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. Ametrine is a combination of amethyst and citrine within a single crystal and was discovered only a few years ago in Bolivia. Such bicolored stones are fairly unusual and can be very striking in appearance, but prices remain rather low. For a brief period, these stones were erroneously thought to be manmade or treated to produce the bicolor effect, but it is now known to occur naturally. Although inexpensive, it is seldom seen and remains something of a collector's item.