text: That would be quartz or SiO2... aka silica or silicon dioxide.\n\nThere are many forms & colors of quartz such as clear, milky, rose, smokey, citrine, amethyst, etc. It is also found as opal which is non-crystalline silica (no crystals).\n\nIt is found as sand grains, as quartz veins, & in rocks such as granite. It is also found alone such as above. SiO2 is also part of a major mineral group called the silicates.\n\nFeldspar is also common but it is one of the most common rock-forming minerals. Feldspar easily weathers & is broken down into clay in warmer wetter climates (ie southern states & the tropics)