The most common and abundant of all the minerals, quartz is a significant component of many igneous, metamorphic, and sedimentary rocks. It is found in an impressive range of varieties and colors; the purple variety is amethyst, which gets its color from small amounts of ferric iron; the yellow variety is called citrine; while the dark variety, smoky quartz, is formed from exposure to radioactive material. The name is a German word of ancient derivation.
