Cat’s eye pezzottaite
Cat’s eye pezzottaite, also known as raspberyl or raspberry beryl, is a rare and unique gemstone that has captivated gem enthusiasts worldwide.
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Raspberyl Cat’s Eye Overview
Pezzottaite, commonly marketed as “raspberyl” or “raspberry beryl,” is a relatively new gemstone species recognized by the International Mineralogical Association in September 2003. Cat’s eye pezzottaite is a caesium analogue of beryl, a silicate containing caesium, beryllium, lithium, and aluminium, with the chemical formula Cs(Be2Li)Al2Si6O18.
Origin and Discovery
This beautiful mineral was named after the Italian geologist and mineralogist Federico Pezzotta. Initially, Cat’s eye pezzottaite was believed to be either red beryl or a new variety of beryl known as caesium beryl. However, unlike true beryl, pezzottaite contains lithium and crystallizes in the trigonal crystal system instead of the hexagonal system typically seen in beryl minerals.
Color and Formation
Cat’s eye pezzottaite ranges in color from raspberry red to orange-red and pink, with specimens often exhibiting vibrant hues. The gemstone was first discovered in miarolitic cavities in the granitic pegmatite fields of the Fianarantsoa province, southern Madagascar. These pezzottaite crystals were generally small, with a maximum dimension of about 7 cm (2.8 inches), and had a tabular or equant habit. Due to their scarcity, only a limited number of pezzottaite crystals have been found, and most are heavily included, containing growth tubes and liquid feathers.
Chatoyancy and Gem Size
Approximately 10% of the rough material has chatoyancy, displaying the “cat’s eye” effect when polished. Most cut pezzottaite gems are under one carat (200 mg) in weight, with larger specimens exceeding two carats (400 mg) being rare.
Cat’s eye pezzottaite from Madagascar
Cat’s Eye Pezzottaite Identification
Cat’s eye pezzottaite has a hardness of 8 on the Mohs scale, similar to beryl. However, its physical and optical properties distinguish it from typical beryl. It has a specific gravity of 3.10, a refractive index of 1.601 to 1.620, and a birefringence of 0.008 to 0.011 (uniaxial negative), all of which are higher than those of beryl.
Physical Properties
The gemstone is brittle and displays a conchoidal to irregular fracture, with a white streak. Similar to beryl, pezzottaite has imperfect to fair basal cleavage. Pleochroism in pezzottaite ranges from pink-orange or purplish pink to pinkish purple, providing a mesmerizing play of colors. The absorption spectrum of pezzottaite, as observed with a hand-held direct vision spectroscope, features a prominent band between 485 and 500 nm. Additionally, some specimens show weak lines at 465 and 477 nm and a faint band from 550 to 580 nm.
Rarity and Sources
The original Madagascan deposits have mostly been exhausted, making the gemstone increasingly rare. Cat’s eye pezzottaite has also been discovered in Afghanistan, although the material was initially mistaken for caesium-rich morganite (pink beryl). Like morganite and bixbite, pezzottaite owes its distinctive color to radiation-induced color centers involving trivalent manganese. Notably, pezzottaite loses its color if heated to 450°C for two hours, but the original color can be restored through gamma irradiation.
Raspberry Beryl Cat’s Eye Effect
In gemology, chatoyancy (also known as chatoyance or the “cat’s eye effect”) is an optical reflectance effect visible in certain gemstones. The term “chatoyancy” is derived from the French phrase “oeil de chat,” meaning “cat’s eye.” Chatoyancy occurs either due to the fibrous structure of a material, as seen in cat’s eye tourmaline, or due to fibrous inclusions or cavities within the stone, as observed in cat’s eye chrysoberyl.
Chatoyancy in Pezzottaite
In the case of pezzottaite, the chatoyancy is caused by needle-like precipitates that align perpendicularly with respect to the direction of the cat’s eye effect. Microscopic examination has shown no evidence of tubes or fibers in the examined samples. Instead, the alignment of needle-like precipitates along one of the three orthorhombic crystal axes of chrysoberyl produces the striking cat’s eye phenomenon.
Cut and Shape for Optimal Effect
This optical effect resembles the brilliance of a spool of silk, with the luminous streak of reflected light always appearing perpendicular to the direction of the internal fibers. To maximize the visibility of the cat’s eye effect, pezzottaite gems are typically cut into cabochons—round stones with a flat base. This shape, with the fibers or fibrous structures aligned parallel to the base, creates the best finished specimens that display a single, sharp band of light that moves across the stone when rotated.
Quality Considerations
Chatoyant pezzottaite gems of lower quality exhibit a more banded effect, similar to that seen in cat’s-eye quartz varieties. Faceted stones generally do not display the cat’s eye effect well, making the cabochon cut the preferred shape for this optical phenomenon.
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