The most common schist that occurs on Earth is mica schist, which essentially consists of quartz and mica, but may also contain some feldspar. Similar to other schists, mica schist is metamorphic and flakes of its component minerals (usually muscovite or biotite) are visible to the unaided eye. Abundant amid Precambrian rocks, a variety of valuable materials may be interbedded with mica schists. For example, ancient Egyptians mined massive quantities of mica schist to find emeralds, which have also since been found in mica schists in Austria and the Russian Urals. Similarly, other types of beryl, aventurine, and even marble, are frequently found amid deposits of mica schist.