Gemology glossary

colored baroque pearls produced by an edible univalue mollusk which has an ear-shaped shell prized for its multicolored mother-of-pearl lining. These are found in America, New Zealand and Japanese waters. Seldom spheroid, they are usually flattened and ear or tooth- shaped

to take up (e.g., a substance) or take in or gain energy from radiant energy (e.g., light)

the pattern of dark vertical lines, bands, or areas (broad absorption), seen when light which has been transmitted or reflected from a gemstone is dispersed into its spectral components and examined using an instrument such as a spectroscope.

see luster

the characteristic bluish-white or milky sheen (schiller) moonstone

when the transition element is present only as a minor impurity in the gem material, the material is allochromatic (allo=other; chroma=color)

general term commonly used for deposits of rock debris (alluvial deposits) which have been transported by flowing water and laid down in stream, river, flood plain or lake beds.

see non-crystalline

see doubly refractive

Archimedes principle states that when a body is immersed in a liquid, then the upward thrust of the liquid on the body is equal to the weight of the liquid displaced. (The determination of specific gravity by the hydrostatic weighing method is based on this principle)

man-made crystals that have no natural counterpart (e.g., the diamond simulants: strontium titanate and YAG)

the enhancement of the appearance of a substance by heating, staining, coating, impregnation, irradiation or laser-drilling

any gem made up of two or more parts which have been cemented or fused together to form a single stone

asterism is a reflection effect, generally four or six-rayed, from aligned fibers or fibrous cavities in a stone which has been cut en cabochon with the inclusions oriented parallel to the girdle plane

strong spangled reflection of light from plates or flakes of another mineral included in the stone (e.g., as seen in aventurine glass, aventurine quartz, aventurine feldspar (sunstone))

one of the elements of symmetry, an axis of symmetry is an imaginary line through the center of a crystal, about which the crystal can be rotated so as to present the same appearance 2, 3, 4 or 6 times in one complete rotation (i.e., a similar face occupies a similar position, more than once in a complete rotation)

name applied to pearls, both natural and cultured, blister or cyst,
which are irregular in shape

basalt, the most common extrusive igneous rock, is a dark, fine-grained rock essentially made up of dark feldspar and pyroxene (with or without olivine). There are a number of varieties which are classified depending upon variations in texture, structure and mineral content

in the orthorhombic, monoclinic and triclinic systems there are three crystal axes of different lengths. Double refraction will occur in most directions, but there will be two optic axes (directions of single refraction) that differ in orientation in each separate gem species. minerals belonging to these systems are known as biaxial. (In the orthorhombic system the optic axes are equally inclined to the 'c' (principle vertical axis), while in the monoclinic and triclinic there is no direct relationship with the crystal axes.)

(also referred to as double refraction/ DR) birefringence is the amount of double refraction of an anisotropic gemstone expressed as the difference between the refractive indices at maximum separation

non-nucleated cultured pearls (often oval or baroque in shape in Japan) farmed around the shores of Lake Biwa in Japan. The pearls are grown in large fresh water mussels by inserting small fragments of mantle into the body of the mussel

general term for a flaw which affects the surface of a gem. A blemish is usually caused by human actions, either while the gem is being cut or while it is being worn in jewelry. Scratches, pits, and abrasions are the most common types of blemish

pearls which are covered on one side only by a nacreous layer which grew over an original irritant in contact with the mollusk shell. When such pearls are extracted, the area which was in contact with the shell is bare of nacre. Accordingly this bare side is usually smoothed off and hidden by the setting

brilliance is the total quantity of internally reflected light that is returned to the eye from the gemstone. Alternatively it can be expressed as the brightness (life) of a gemstone as determined by the amount of light reflected back to the eye from the back of facets of the stone

brittleness is defined as a weakness in a gemstone which despite its hardness, makes it susceptible to fracture, damage (e.g., heat treated zircon)

see chatoyancy

one of the elements of symmetry, a center of symmetry is present when every face of the perfect crystal is diametrically opposite a similar face on the other side of the crystal (i.e., every face has another face that is similar, opposite and parallel)

a reflection effect from oriented parallel fibrous inclusions or (cat's eye) cavities. The single streak of light appears at right angles to the direction of the inclusions and it is best seen under a single overhead light source with stones that have been cut en cabochon

the instrument consisting of a combination of filters, is so constructed that it allows only two narrow sections of the spectrum to be visible through it: the deep red (near 690nm) and the yellow green (near 570nm). It is useful in indicating the presence of chrome or cobalt in certain gems, for the detection of (chrome) dye in stones and for separating some gem materials from their common imitations. Reminder: Use as warning sign only. Proof is dependent upon other tests

(coloring agent). The part of the molecule that is responsible for a compound's color. It is present as an essential constituent in the chemical composition in idiochromatic stones or as an accidentai impurity in allochromatic stones

term used to describe the relative freedom from any internai defect or irregularity (inclusions) in a gem. The definition may also be extended to include surface defects (blemishes) when considering the clarity grade of the stone

roughly parallel bands of color seen on the surface of crystals or in the internai structures of stones (indicative of growth phases caused by differences in available chemicals, liquids, temperature, pressure and other factors)

a visual perception of the constituents into which white light can be separated (red, orange, yellow, green, blue, violet) as well as the sensation of purple which is not found in the sun's spectrum. Color is described by hue, saturation and tone. The body color of a gemstone is derived from the light by which it is seen as well as being due to the absorption of certain wavelengths of visible light. Color can also be caused by dispersion, interference of light and fluorescence

color zoning develops during the growth of the crystal due to inconsistent concentrations of impurities. The variations in color (bands or zones) are normally seen to follow the existing crystal faces

Selective absorption of light due to the presence of the transition elements (chrome, cobalt, copper, vanadium, manganese, iron, nickel, titanium) in idiochromatic and allochromatic stones

compact has a similar meaning to massive and is applied particularly when the crystals show no sign of structural particles (e.g., fme-grained jadeite jade)

see assembled gemstone

pink pearls: pearls obtained from the great conch, a univalve mollusk found off the coast of Florida, gulf of California and Mexico. These pearls are normally pink (pink orange or white) in color and are characterized by a lack of nacreous coating

see fracture

the recrystallization of the mineral content of pre-existing rocks in or near the contact zone due to presence of magmas

a type of crystal twinning in which the individuals are in contact along a common plane (the twin plane). The two parts of such crystal are so related that if one part is rotated through 180° (a half turn) about an axis (the twin axis) then the two halves would make the shape of the individual crystal. The contact twin exists in two forms: single contact twin—where 2 halves of a crystal are in reverse order so that if one half is rotated 180° about the plane of join, the form of the normal crystal is obtained; repeated, polysynthetic or lammelar twins—a type of crystal twinning comprising a series of contact twins often in very thin plates. These are arranged in opposite orientation to their immediate neighbors (e.g., as found in corundum and feldspar). See also interpenetrant twins

the angle of incidence where a ray of light which is traveling from an optically denser to rarer medium is refracted at 90° to the normal (i.e., it skims the surface of the two media in contact). Note: any further increase on this angle of incidence would cause the refracted ray to turn back into the original medium where it would abide by the laws of reflection (i.e., it would be totally internally reflected.)

(crypto=hidden). Term used to describe material consisting of an immense number of small crystals often sub-microscopic forming an unbroken mass. The sub-microscopic crystal aggregate can appear amorphous to the naked eye

a crystal is a chemically uniform solid with an ordered internai atomic structure and external form bounded by symmetrically arranged plane (flat) faces

a crystal form consists of ail those faces which are similarly related to the crystallographic axes (e.g., pyramid, prism, pinacoid, dome)

the crystal shapes (form, plus surface characteristics) in which minerals usually occur are called their crystal habits

crystal inclusions are identifiable by their having relatively angular corners and relatively straight edges. Uneven edges and slightly rounded forms are also met with due to reabsorption of the inclusions. (crystal inclusions usually show up in relief against the host background when viewed under darkfield illumination. They can be checked by using crossed polars.)

crystal symmetry refers to the balanced pattern of the atomic structure. It is the precise reproduction (repeat appearance) of similar crystal faces (edges, corners) that indicate the ordered internal arrangement of the substance.

crystalline material by definition is a mineral solid whose ions, atoms or molecules are arranged in an orderly fashion. In practice, the term is often used to describe substances possessing the regular structure and directional properties but not necessarily the external geometric shape

these are imaginary lines of indefinite length running through the
ideal crystal in certain definite directions in relation to the symmetry of the crystal. They intersect in the center of the crystal at a point called the origin. (They are lines of reference from which can be measured the relative distances and inclinations of the various faces.)

cultured pearls are pearly formations entirely or partially caused
by the intervention of human agency into the interior of the productive mollusks. This applies to cultured pearls whether with or without a nucleus

a pearl which has formed within the body of the pearl mollusk and is spheroid in shape

tree-like or moss-like crack filling (where inclusions of different material may fil a break - typically iron oxide.)

to deprive of glassy luster and transluceny. (In the case of the glass product called meta jade - is by the development of incipient crystallization)

the dichroscope is an instrument which allows the pleochroic effects of gem minerals to be observed, two colors or shades of color at a time

the diffraction of light is a specialized form of interference of light in which white light is broken up (split) into its component colors as the light passes through a small aperture or when light waves pass an edge between differently refracting substances (e.g., the play of color seen in precious opal)

dispersion is the separation (breaking up) of white light into the spectral colors by refraction (the bending of light) as light passes through two inclined surfaces. In gems it is often referred to as 'fire'

anisotropic - minerals exhibiting directional optical properties (e.g., stones in the tetragonal, hexagonal trigonal, orthorhombic, monoclinic and triclinic systems)

term used for the entire range of wavelengths of radiant energy from the longest radio waves through the extremely short cosmic wavelengths

the elements of symmetry are the manner of classifi-cation for the seven crystal systems. They are devices to enable us to separate and describe the various types of crystal shape. They include a plane of symmetry, an axis of symmetry and a center of symmetry (see definitions under individual listings)

eluvial deposits consist of rock debris (gravel) which has been eroded (weathered out) from the host rock and remains in situ (in place) without undergoing transportation in rivers

to give off or out (e.g., light)

the molecular structure of quartz which gives rise to the left and right hand spiral growth of quartz, and to its circular polarization. (Note: The placement of small auxillary faces which are frequently visible on the external form help identify the left or right handedness of the crystal)

see artificial treatment

post contemporary inclusions. Those which occurred after the formation of the host crystal. These include various types of fissures and mineral inclusions formed by exsolution (e.g., rutile silk in corundum); not to mention oil/opticon, etc., remnants in fracture filled gems.

see exsolved

the unmixing of minerals. Some pairs of minerals form solid solutions at high temperatures and become unstable at lower temperatures. When these cool slowly one mineral may be forced out when the host structure cools and contracts, literally squeezing impurities into vacant spaces where they crystallize (e.g., rutile needle (silk) in corundum)

extinction is the absence of light returning to the eye. This causes the stone to appear dark and lacking in billiance

faceted gems have a crown (top portion above the girdle), a table (the center facet of the crown), a girdle(the section of the stone with the greatest perimeter or external boundary), a pavilion (the bottom part below the girdle). All other facets may vary in their size, frequency or even existence depending on the cutting style used. For example, in the standard step-cut (emerald cut) stones the culet (small facet at the base of the pavilion) is replaced by a keel line (long line extending along the center of the bottom of the pavilion)

see parting

a plane of minute cavities which normally appear as a multitude of fine rod-like sections of intercommunicating channels (usually liquid filled).

name given to material where the constituent crystals are needle-shaped (e.g., asbestos, nephrite jade)

these may also may be known as healing feathers and consist of interconnecting networks of liquid-filled channels and droplets resembling a fingerprint. Usually the result of mineral solutions gradually filtering into a pre-existing crack in an already formed crystal, and healing the fissure. The other situation where such features can occur is during heat heat treatment. Fingerprints are common in treated rubies

finish refers to the faceting quality of the gem and is judged by the degree of shine, regularity, and flatness of each facet

fluorescence is the emission of visible light by gemstones that have been exposed to visible or invisible radiations of shorter wavelength (higher energy)

fracture is an irregular chip or break in a stone

a precious stone of any kind, cut and polished for ornament (pearl is a notable exception to the latter requirement). A gem by definition is one that possesses beauty, durability and rarity

the term glass is applied to substances which are created by the melting and rapid cooling of mineral substances, resulting in the formation of amorphous material. Glass can be artificial (manmade, sometimes referred to as paste) or more rarely, natural e.g., obsidian (volcanic glass), tektites (thought to be meteoric splash products)

granite is a light colored, coarse-grained intrusive igneous rock made up mostly of quartz, feldspar and mica

lighter and darker bands in straight lines at angles that follow the structural pattern of the mineral

the characteristic shape in which a mineral most commonly occurs. Habit not only includes the external form commonly adopted by the mineral but also typical surface effects as well

see fracture

crystal inclusion surrounded by a stress crack(s). These may also look like a butterfly with two wings emanating from the included crystal (which is usually zircon)

hardness is the power a mineral has to resist scratching (abrasion). See also Moh's scale

healed cracks (cracks or cleavages) where a liquid entered the break and the crystal grew back together. Often portions of the fluid are trapped in the cavities and channels causing feathery or fingerprint-like patterns (sometimes iron-stained)

hue refers to the name of the color itself: the pure spectral sensations of red, orange, yellow, green, blue, violet and the variations of these. Hue also refers to the sensation of purple (between red and violet), which is not found in the sun's spectrum

rocks laid down, usually in the last stages of igneous consolidation from magma rich in water and other volatile substances. The hydrothermal veins of mineral rich solutions filter through gaps and fissures in sedimentary rocks and form crystal lined cavities in the rock

when the transition element is an essential constituent in a gem material, the gem material is idiochromatic (idio=self, chroma=color)

small glass cell for containing suitable liquids. The principle behind using an immersion cell is that the refractive index of the liquid is to the stone, the easier it will be to see into the stone. Almost any colorless liquid, even water, will cut down surface reflection, and make viewing inclusions much easier

material encountered in the position where it was originally formed or deposited is said to be found in situ (literally in place)

general term for any defect or irregularity within a gem. Inclusions can be divided into three different types: solid—both crystalline (e.g., zircon in sapphire) or non-crystalline (e.g., natural glass in peridot) are possible. Cavities—these may form during the host gems growth (primary inclusions), or later (secondary inclusions). Either type may be filled with any combination of liquid, gas and or solid. Those with two or more phases are termed multiphase. Growth phenomena—color zoning and twinning are both examples of growth phenomena

infrared means below red and this range of radiation begins with wavelengths that are no longer than the 700nm of red light. Infrared radiation is generally known as heat because it produces a sensation of warmth when it strikes our skip

where two rays traveling in the same path, but out of phase (step), mutually interfere with one another causing either total extinction of light or the pre-dominance of one or more colors

these are optical effects seen when double refractive minerals are viewed through parallel faces in convergent beam of polarized light. They afford useful information concerning the optical character of the stone

a type of crystal twinning in which two individuals have so grown together that they appear to have penetrated one another (often producing cross or star stones)

iridescence or play of color describes the color or series of colors produced by interference or diffraction when light is reflected either from thin films or from the unique structure of precious opal

the substitution of one chemical element for another of the same valency in the chemical composition. This results in wide variations in the physical properties of the members of the series. Valency: of similar chemical nature and ionic radius

see singly refractive

naturally occurring non-nucleated freshwater pearls which form in a mollusk when it is returned to the water after removal of a crop of non-nucleated cultured pearls. These are not to be confused with marine seed pearis which are usually much smaller. The freshwater `keshi' are only so-called because they likewise are not grown from an artificial nucleus. Sometimes referred to as `seedless' pearls to make the distinction with non-nucleated. See also Biwa pearls

the Japanese call seed pearls `keshi' (poppies). The association here is between the tiny size of poppy seeds and the very small naturally formed pearls

a polariscope fitted with a strongly convergent lens which helps facilitate the viewing of interference figures

caused by polysynthetic twinning. Can appear as blinds or transparent fish-line. They may also appear to meet in angles (e.g., in corundum where directions meet)

ground material (composed of rock fragments and vegetation) which is primarily derived from decomposed basait (a fine grained dark volcanic rock rich in iron and magnesium, which is very open to the effects of chemical weathering in a tropical climate)

general term for the giving off of visible `cold' light by a substance upon acquiring surplus energy in one form or another. The five principles luminescent phenomena are: chemi-luminescence (result of chemical change), tribo-luminescence (produced by friction), thermo-luminescence (produced by heat), photo¬luminescence (produced by visible or invisible radiations of higher energy/ shorter wavelength) and cathodoluminescence (results from excitation with an electron beam in a vacuum chamber)

luster is the quantity and quality of light that is reflected from the surface of the stone. The following terms are used to describe gemstone lusters: metallic: high luster shown by opaque metallic minerals. adamantine: high surface reflectivity. vitreous: glass-like luster typical of a majority of gems. resinous: typical luster of resins (soft with low R.I). waxy: almost matte surface (sometimes called greasy). greasy: slightly oily looking resulting from the scattering of light by a microscopically rough surface. pearly: luster of pearl/mother-of-pearl. silky: fibrous luster of silk

composite cultured blister pearls in which the original nucleus has been removed and replaced by a smaller bead cemented in, and a dome-shaped piece of mother- of-pearl glued to its base to cover the area lacking in nacre

molten (liquid) rock

malleable means being capable of being extended or shaped by beating with a hammer or by the pressure of rollers

a man-made product by definition is one that has been synthesized in a laboratory. (to synthesize is to put together or combine elements into a complex whole). The end result may be a synthetic gemstone (i.e., one that has a natural counterpart) or an artificial gemstone (i.e., one that has no natural counterpart)

term used to describe material which possesses the definite internai structure of a crystal, but which does not show the outward geometric form. This can be composed of one (e.g., rose quartz) or more crystal units (e.g., jadeite)

see luster

the term used for material which has suffered a break-down from a crystalline to a partly amorphous state; common in minerals containing radioactive materials (e.g., low type zircon)

metamorphic (literally, changed in form) rocks are formed from pre-existing rocks primarily by the action of pressure, heat and/or the introduction of new chemical substances

one metric carat=one fifth of gram (0.20gr) or 200 milligrams (mg). A metric carat is the unit of weight for ail gemstones and cultured pearls. The weight is expressed to two decimal places and the decimals are often loosely called 'points', i.e., 1 metric carat=100 points

an optical instrument consisting of a combination of lenses which produces a magnified image of an object. Some of the more important uses of magnification: inclusion study (natural vs. man-made; mode of formation/crystal growth); surface examination(quality of cut(symmetry), polish(conditions of facets, firemarks); detection of composite stones; identification of artificial treatments; detect vulnerability to damage (e.g., presence of fracture and/or cleavage); detect doubling (SR vs.DR, also may indicate amount of birefringence)

a mineral is a naturally occurring inorganic substance having a chemical composition and physical properties which are constant within narrow limits. Its structure is usually crystalline

monochromatic light is light of one wavelength only. The standard yellow monochromatic light used for Refractometer measurements is derived from a sodium vapor lamp. This actually consists of two very closely spaced emission lines, whose mean value is 589.3nm

inclusions that may contain liquid and gas and/or crystals

a secretion produced by the mantle of some mollusks which forms the iridescent layers of both the pearl and the mother-of-pearl surface inside the mollusk shell. Nacre consists of crystalline carbonate of lime and an organic material called conchiolin

unit of measurement for the shorter wavelengths of the electromagnetic spectrum (radiant energy). 1 nanometer = 1 millioneth part of a millimeter (1/1,000,000 nm) e.g., visible light falls between 700nm(red) and 400nm(violet)

term used for a style of cut chosen for maximum weight retention. These deep cut stones with large pavilions often require
recutting to attain better proportions

cavities within the gem having a crystalline shape. These cavities were created when certain areas grew at a faster rate than other areas, forming and enclosing a void or where the original crystal inclusion has been dissolved out (negative crystals often appear lighter than their host under darkfield illumination)

a negative inclusion may be gas or liquid filled or contain a small solid inclusion (see also negative crystal)

if the stone under test on a refractometer has an R.1 above the range of the instrument, no shadow edge will be seen (except that of the contact liquid)

minerals and other materials are said to be non-crystalline or amorphous (literally, without form)when they Jack an orderly internai atomic structure and a geometrically regular outer appearance(e.g., the mineral substances, opal and glass, as well as organic materials)

non-white light(i.e., colored)is described in terms of its hue(i.e., the color of its pre-dominant wavelength), saturation(depth or strength of dominant color) and tone (shade—light to dark)

in optics ail angles are measure from the normal, an imaginary line drawn at right angles(90 point where the ray strikes the surface and not from the surface itself

a direction of single refraction within an otherwise doubly refractive crystal is known as an optic axis

see optic nature

a glass sphere fitted to a short rod which acts as a condensing lens to help resolve interference patterns seen along an optic axis in anisotropic material

optic nature of a mineral: minerals are divided into uniaxial, biaxial and isotropic(optic character). Uniaxial and biaxial minerals are further subdivided into those which are optically positive and those which are optically negative(optic sign)

see optic nature

optical density is a complex property which demonstates itself in the slowing down of light. (see also refraction)

organic products are those materials produced by the activity of living organisms

the iridescent surface sheen of gem pearl and mother-of-pearl. It is caused by interference of light at thin films (the thin plates of the nacreous layer)and by diffraction from the fine edges of the plates

use of this term is restricted to describing natural pearls found in sait water mollusks of the Persian Gulf. However, it has been the more general trade custom to class ail saltwater natural pearls as oriental

parting is a breakage that occurs along a plane of weakness, rather than along planes parallel to crystal faces, or possible
crystal faces, as in cleavage (it is also known as false cleavage)

the pearl grain is the standard unit for pearl weighing 1 grain=0.25 carat (1 ct=4grains)

pearls are natural formations secreted accidentally and without the aid of any human agency, in the interior of mollusks. They are composed of an organic substance (a scleroprotein named conchiolin) and of calcium carbonate (usually in the form of aragonite)arranged in concentric layers, the outermost of which are mostly nacreous

see luster

igneous rock of coarse gain often containing rare minerals, including large crystals of gemstones (e.g., beryl, chrysoberyl, feldspar, quartz, spessartite garnet)

phosphorescence is in effect delayed fluorescence (i.e., it is an afterglow). It is the continued emission of visible light after the source of energizing radiation has ceased

photoluminescence is a collective term for fluorescence and phosphorescence. It is the effect exhibited by certain materials of producing visible light upon being exposed to short wavelength radiation (e.g., electromagnetic radiations in the visible (blue), ultraviolet and x-ray part of the spectrum)

small particles found as inclusions. When these are present in large numbers they may be described as a cloud and the transparency of the stone may be impaired

placer deposits consist of surface concentrations of valuable minerals of high specific gravity (and durability)which have formed by weathering of the parent rock and, in most cases, subsequently been transported by stream or wave action

one of the elements of symmetry; a plane of symmetry is an imaginary plane which divides a body into two parts so that each part is the reflected (mirror)image of the other part (i.e., each part is the exact inverse counterpart of the other)

a general term for a number of organic man-made substances (usually a resin based polymer)which can be extruded or molded by heat and/or pressure

the term play of color describes the series of colors seen when light is reflected from thin films or from the singular lattice-like structure of precious opal. It is produced by diffraction (the breaking up of white light into its component colors as light passes through a small aperture)which is a specialized form of interference of light

pleochroism(literally, many colored)is the general term used to describe the different directional colors which may be seen in certain colored stones that split light (i.e., are doubly refracting)due to their internal crystal structure. It includes dichroism(two colored) and trichroism(three colored)

the polariscope is an instrument consisting of two units for the production of plane polarized light arranged with a rotating stage between them. The device tests only whether or not material is singly or doubly refractive

singly refractive—material remains dark throughout a 360° turn. If the material is under internai strain it may show anomalous (false)double refraction(A.D.R), usually in the form of wavy bands or irregular patches. Double refractive—stones cut from a single crystal have four positions of light and four positions of dark. Crystalline aggregates or heavily included DR material may appear ail light throughout a 360° turn. Interference figure—in the case of D.R material, if the conditions are right and an interference figure is noted, it is also possible to determine whether the stone is uniaxial or biaxial. (optic sign—once having found the interference figure, it is then possible to determine whether the stone is positive or negative with the aid of
accessory plates supplied specially for that purpose)

minerals which are aggregates of small crystals are termed polycrystalline. These small crystals can be discerned using magnification, and sometimes by the eye alone (e.g., almost ail jadeite jade)

see lammelar twins

milky material lacking in play of color, composed of silica spheres of unequal size thereby causing the light to be scattered rather than diffracted

pre-existing inclusions: those which were present before the host crystal began to form and were absorbed 'ready made' into the latter (e.g., solid particles and small crystals. They tend to be irregularly strewn throughout and are seldom related to the host crystal's structure

a pseudomorph (false form) is a mineral which takes the form (shape) of another mineral(or organic substance)due to heat and/or pressure or chemical process, e.g., tiger's eye(quartz replacement of crocidolite, an asbestos mineral); wood agate(a quartz replacement of wood)

reflection is the return by a surface (whether internai or external) of some of the light which falls on that surface. See also Snell's Laws

refraction is the change in direction in which light travels when it passes from one media to another of different optical density (except when striking the common surface of the two media at 90°). As light passes from a rarer to a denser medium (e.g., from air into a stone) it is bent towards the normal, conversely as it passes from a denser to a rarer medium it is bent away from the normal. Optical density is a complex property which demonstrates itself in the slowing down of light. See also Snell's Laws

the R.I is a simple ratio comparing the speed of light in air to the speed of light in a given medium. The standard used for measuring refractive index is air (strictly a vacuum); thus the R.I of air is considered to be 1.00

the recrystallization of the mineral content of pre-existing rocks at increased temperatures (700°-2000°C) and under pressure

See specific gravity (S.G)

residual wavelengths are those wavelengths not absorbed by the material which combine to give us the perc-eived color of an object

See luster

a rock consists of mineral particles which have grown or have cemented together by chemical processes or been bonded together by heat or pressure

refers to the quality or intensity of color seen in the brilliancy flashes of transparent faceted stones

See sheen

sparkle; the giving off of short, bright flashes of light

sectile refers to the materials capability of being peeled by the knife with a smooth cut

those formed by the build up of material (sediments) which originated from the physical and chemical breakdown of pre-existing rocks at the earth's surface

very small pearls (less than v. grain / approx. 2mm) that are naturally formed in the soft tissue of the mollusk as a result of the invasion of grains of sand, free floating eggs, parasites or other foreign bodies. Seed pearls are usually irregular and flat, but when rare round ones are found, they are separated out for use in jewelry. See also Keshi pearls

selective absorption of light is the suppression or absorption of certain wavelengths from the light passing through an object or reflected off its surface. The color of the material results from the remaining (residual) wavelengths that reach the eye

sheen is a shimmering or diffuse(spread out) reflection effect caused by light being reflected from inclusions or structural features within a stone. The term includes chatoyancy, asterism, adularescence, aventurescence, opalescence, iridescence (labrodrescence and play of color in opal)

a series of fine parallel needles(frequently rutile)

also referred to as isotropic. minerals which exhibit the same optical properties in ail directions(e.g., stones in cubic system and amorphous material)

when a light ray passes from one medium into another there exists a definite ratio between the sine of the angle of incidence and the sine of the angle of refraction. The incident ray, the refracted ray and the normal (at the point of incidence) are ail in the same plane. The constant ratio between the angle of incidence and refraction of any two media in optical contact was called by Snell the refractive index (R.I)

giant cultured pearls, cultured in silver-lipped pearl oysters with a diameter of 12 or even 16mm. These big white pearls earned the name South Sea because they were originally produced by the Japanese in Microne-sia and Indonesia. Today these pearls are more frequently produced in Australia and Burma, with Indonesia, the Philippines, Papua New Guinea and Thailand as less important sources. To date there has not been a successful artificial spat collection of the silver-lipped pearl oysters, which means that the supply of the mother oysters is limited and prices accordingly high

the S.G or relative density of a material is the ratio between the weight of a substance and the weight of an equal volume of pure water at 4°C (water's maximum density) and at standard atmospheric pressure. S.G is related to the chemical composition of the material as well as the inter-atomic bonding (in effect, it is influenced not only by the atomic weight of the various component elements but also the manner in which the atoms are placed together)

an instrument which separates a ray of light into its constituent wavelengths or spectrum colors by means of a prism or diffraction grating. The spectroscope allows one to see which specific sections (lines or bands) or areas(broad absorption)of white light are absorbed by a gemstone by dispersing or spreading out the residual wavelengths which have been transmitted or reflected from the stone under test(not ail stones show a spectrum and not ail spectrums seen are diagnostic but absorption bands may indicate coloring agent or the presence of certain elements)

see fracture

symmetry refers to the regularity or balance of the shape of the gem in both the face-up position and in profile

contemporary inclusions consisting of substances which were presented at the same time as the host crystai(e.g., mineral solids; liquids in cavities and healing cracks; zonal bands; liquid inclusions; traces of growth such as color zoning and twin formation. There is frequently an ordered intergrowth between the syngenetic inclusions and host crystal so that the two may bear a structural relationship governed by crystallographic laws

a synthetic product has the same chemical composition, atomic structure and physical properties as its natural counterpart(e.g., synthetic emerald, synthetic corundum, etc.)

tenacity or toughness is the resistance of a gem to breaking or fracturing. It is related to the minerals ability to absorb shocks without disruption of the internai structure. Terms met with in this regard include brittle, sectile, malleable, flexible and elastic

viewed under reflected light these thin films have an iridescent appearance typical of a thin layer of oil on water or the multicolored surface of a soap bubble

a liquid-filled cavity containing a bubble and a crystal

tone refers to the relative lightness or darkness perceived (whether of a hue or of white to black)

total internal reflection occurs where a, ray of light passing from a denser to a rarer medium at an angle greater than the critical angle is returned to the denser medium where it obeys the laws of reflection

see tenacity

name applied to certain metallic elements which due to their atomic structure, effect the selective absorption of light in idiochromatic and allochromatic stones. They are chrome, cobalt, copper, vanadium, manganese, iron, nickel and titanium

to pass(e.g., light)from one medium to another; to send out (e.g., a signal) or send on (e..g., from one person to another)

the freedom with which light is passed or transmitted through a stone. The different degrees of transparency are given as: transparent (TP)—a transparent object reflects and absorbs a small amount of the light which falls on it, but allows most to pass through. An object viewed through the stone appears clear and distinct. Sub-transparent (S-TP)—a considerable amount of light is transmitted through the stone, but only the outline of an object on the other side of the stone cannot be distinguished. Sub-translucent (S-TL)—only a very small amount of light can pass through the gem at the thin edges. Opaque (0)—ail of the light which falls onto an opaque object is either reflected or absorbed. No light passes through the material, even at thin edges.

a heat-wave or roiled effect—wisps and swirls of color

a twin crystal is a crystal which is composed of two halves of the same crystal or two or more crystals of the same species which have a direct crystallographic relationship to one another and have grown together in a symmetrical manner(see contact twins and interpenetrant twins)

a liquid-filled cavity containing a bubble or a crystal

ultraviolet means beyond violet. This range of radiation begins with wavelengths that are shorter than the 400nm of violet light

in the hexagonal, trigonal and tetragonal crystal systems there is one optic axis (direction of single refraction): minerals belonging to these systems are known as uniaxial (in uniaxial stones the optic axis is parallel to the principal vertical 'c' axis)

the unit cell of crystal structure which is the smallest part of a crystal which still possesses the characteristic properties of the crystal is termed a unit cell. The manner in which the unit cells are stacked by mother nature determines the external appearance of the crystal

see feather inclusion

visible light is a form of radiant energy which gives rise to the sensation of sight. Any wavelength of electromagnetic radiation that is approximately between 400nm and 700nm is visible to the human eye as light

see luster

a wavelength is defined as the distance between two consecutive peaks of that wave

see luster

white light (mixed light)is composed of an approximately equal mixture of all the colors or wavelengths that make up the visible spectrum (red, orange, yellow, green, blue and violet). The color of light varies with depends upon the wavelength. Red waves have the longest wavelengths (700+nm) and this diminishes through the spectrum to violet wavelengths, which have the shortest (400nm—about 1/4 the length of the red rays)

transparent area of low color intensity caused by light leakage which permits the background behind the stone to be seen (read-through effect)

see halo inclusion

see color bands