Though pure quartz is colorless and transparent or translucent, many different varieties of quartz exist. Common colored varieties include citrine, amethyst, and smoky quartz, but quartz can be found in almost ever hue. Though it is one of the most abundant minerals on Earth, quartz comes in a large variety of shapes, colors, structure and, as a result, rarities. Citrine, for example, is rarely found naturally and most of the citrine found on the market is actually heat treated amethyst. Heat treatment for quartz and other gemstones is common and has been used for years to bring bright color to gems. This wide range of looks makes the quartz group far reaching, ranging from deep, dark hues to the clear, lustrous look of fine glass. For centuries, pure quartz was thought to be ice that was permanently frozen after a long period of time. This clear, colorless variety of quartz is commonly referred to as rock crystal.