Amethyst is easily the most popular type of crystalline quartz in fine jewelry. One of the planet’s most ubiquitous minerals, quartz is also the source of citrine. For thousands of years, purple amethysts have been fashioned into everything from beads to amulets. In the 19th century, Victorian Era jewelers set amethysts into their necklaces, earrings, and brooches. By the 1930s, the stones had become to go-to gems in Art Deco jewelry.

Thought to ward off drunkenness, amethysts range in color from deep violet purples to pale lavenders. Vividly colored stones are called Siberian amethysts regardless of where they were mined. The Ural Mountains continue to be one of the primary sources of the best gems, which shift from purple to red under artificial light. Stones with bluish tints are found in Madagascar and Hungary, while reddish ones are common to mines in North Carolina and Mexico.

Lightly colored stones, which are sometimes referred to as Uruguayan amethysts, are often heat-treated to mimic the look of a dark citrine. In fact, most of the citrine on the ma...rket today began life as a pale or inclusion-riddled amethyst. Heat-treated amethyst is also marketed as topaz quartz, which is as meaningless a designation as it sounds. Stones of poor quality are sold as Bahian amethysts.