The emerald green color is the most valuable of this group. These stones have perfect cleavage and can fade in sunlight. Quartz is composed of silicon dioxide, has a hardness of 7, and comes in many varieties. Rock crystal is colorless. Smokey quartz is brown to black.
Amethyst is violet and is sometimes referred to as purple quartz.
Citrine can vary from a light yellow to a gold-brown color. Prasiolite is a leek-green variety. Rose quartz varies from a pale pink to a strong pink. There are many other varieties that are usually made into cabochons such as chrysoprase, heliotrope or bloodstone, tiger s eye, hawks eye, aventurine, prase, blue quartz, chalcedony, cornelian, sard, jaspers, and agates. The same stone can display three different colors each one from a different direction.