Amethyst is a purple variant of quartz (yellow = citrine, green = prasiolite), with its color coming from iron impurities. Its name comes from the Greek “a-“ (not) and “methystos” (intoxicated), stemming from the belief that the stone protected against intoxication. It was also thought that the stone would make a person more clear-headed. Amethyst can be found world-wide, with large deposits found in Brazil, South Korea, Austria, Russia, and numerous other countries. Before large deposits were found, this stone used to be lumped with diamonds, rubies, and sapphires for its rarity and preciousness. With a hardness of 7 on the Mohs scale, it makes a good gemstone – fortunate, as it is the birthstone for February. Cutting the stone properly can be challenging. The crystals do not often have a uniform concentration of color, but vary in shades of purple and mauve. These crystals are often found in geodes – some of which are massive in size.
Chemical Formula: SiO2
Kunz, George F. The Curious Lore of Precious Stones. New York City: Halcyon House, Blue Ribbon, 1930. Print.
Discovered in 1967 in Tanzania, tanzanite (named by Tiffany & Co.) is one of the more recent birthstones for December (as of 2002, via the American Gem Trade Association). While the stone may be light violet in some cases, darker shades of periwinkle with hints of purple are sought after for higher gem quality. A blue variety of the gemstone zoisite, tanzanite has a Mohs harness of 6.5-7 – not particularly tough for a gemstone. Corundum (rubies and sapphires) sit at a healthy 9.0 on the Mohs scale.
The blue color is caused by trace amounts of vanadium within the ziosite – much like trace elements can also cause various colors of diamonds. The blue can be enhanced and brought out in the stone by careful addition of heat. Tanzanite crystals are also pleochroic – meaning from different angles they exhibit different colors. The same crystal from one direction may look blue, and another direction look red or brown – thus faceting the stone can be challenging.
The chemical composition of tanzanite is: Ca2Al3(SiO4)3(OH)
Newman, Renee. Exotic Gems: How to Identify and Buy Tanzanite, Ammolite, Rhodochrosite, Zultanite, Moonstone & Other Feldspars. International Jewelry Publications, 2010.