Natural amethyst is a purple variety of crystalline quartz (SiO2). It owes its violet color due to natural irradiation of iron impurities. In some cases in conjunction with transition element impurities. The presence of trace elements result in complex crystal lattice substitutions. Also, the hardness of the mineral is the same as quartz. Thus it is suitable for use in jewelry.
Amethyst occurs in primary hues. From a light pinkish violet to a deep purple. It may exhibit one or both secondary hues: red and blue. Siberia, also Sri Lanka, Brazil and Asia are the sources of the best varieties. Deep siberian is the ideal grade name. It has a primary purple hue of around 75 / 80%, with 15 / 20% blue. Depends on the light source. Green quartz is also incorrectly called green amethyst. It’s not an appropriate name for the stone. The proper terminology is prasiolite. Other names for green quartz are vermarine or lime citrine.
The color of natural amethyst is often laid out. From stripes parallel to the final faces of the crystal. Also, one aspect in the art of lapidary involves correctly cutting the stone. Color zoning makes the tone of the finished gem homogeneous. Often, only a thin surface layer of violet color is present in the stone. The color is not homogeneous makes for a difficult cutting.
The color is the result from substitution by irradiation of trivalent iron (Fe3+) for silicon in the structure. In the presence of trace elements of large ionic radius. Also to a certain extent, the quartz color can naturally result from displacement of transition. Elements even if the iron concentration is low. Natural stone is dichroic in reddish violet and bluish violet. But when heated, it turns yellow-orange. It can also tunrs to yellow-brown, or dark brownish. It may resemble citrine, but loses its dichroism, unlike genuine citrine.
Amethyst can fade in tone if overexposed to light sources. It can also be artificially darkened with adequate irradiation.
Amethyst is a semiprecious stone and is the traditional birthstone for February.