Quartz is one of the most abundant minerals on Earth and is found in many different forms almost everywhere. Cryptocrystalline quartz often displays exotic color bands, swirls, and other patterns. Crystalline quartz is quartz that occurs in distinct crystals. It occurs in a number of familiar varieties distinguished by color.
Amethyst is purple quartz and is one of the most popular gemstones.
When exposed to strong sunlight for extended periods, amethyst may fade in color. Recently, gem markets have been flooded with inexpensive synthetic amethyst. Although such synthetics can usually be identified by testing for twinning, routine testing is not cost-effective, and some new synthetics do not show the twinning effect. Citrine is yellow to brown quartz. It varies from pale yellow to rich golden yellow to dark orange.