It contains fine black grains of mica and, occasionally, gems like garnet. Quartzite is a tough, crystalline rock formed by the metamorphism of sandstone. Marble is a metamorphic rock formed by the re-crystallization of limestone. Most of our materials, from building bricks to diamonds, are minerals from the Earth's crust. Gems and crystals such as diamonds, rubies and quartz often form in geodes, gas pockets in cooling igneous rock.
Amethyst is a purple or violet transparent variety of quartz.