Regional metamorphism is when rocks are crushed beneath mountains by the moving plates of the Earth's surface. This turns mudstone and shale to slate, then to schist and then to gneiss as it is squeezed harder and harder. Schist is formed when slate made from shale is crushed even harder. 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.