A quartz crystal cluster from Tibet. Quartz is the second-most abundant mineral in the Earth's continental crust, after feldspar. There are many different varieties of quartz, several of which are semi-precious gemstones. Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent (clear) or translucent. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others.Photo credit: Noodle snacks