Citrine is a a member of the quartz family. Natural citrine is often a pale yellow or orange. Yellow quartz is relatively rare. However, in the 18th century it was discovered that heating smokey quartz or amethyst very carefully could change the gems color from purple to yellow. Today, much of the citrine found in jewelry has been heat treated to create rich colors such as yellow-brown, reddish brown, golden orange, and orange brown. Citrine gets its name from citrus, for its resemblance to lemon. However, the most valueable citrines are not lemon colored but rather those with a rich gold hue.
In nature, sometimes amethyst deposits are subjected to unusually high temperatures. This can create a bi-colored quartz called amertine which combines the colors of amethyst and citrine. The gemstone is half purple, and half golden yellow. A similar effect can be created in the laboratory by heating amethyst. When amethyst is exposed to any high heat it's color will change.
The origin of the tradition of birthstones is sometimes attributed to the Breastplate of Aaron, the high priest of Hebrews. The breastplate of the high priest was a ceremonial garment set with 12 gemstones, each stone representing one of the twelve tribes of Israel. The twelve gems corresponded to the twelve months of the year, and the twelve signs of the zodiac.
Citrine is the modern birthstone for November, and the planetary stone for Virgo.
Citrine has a Mohs scale hardness rating of 7.