Tourmaline crystals in quartzite. Close-up of tourmaline crystals (pink) embedded in a quartzite host rock. Tourmaline has a very complex and variable composition. Clear and pure crystals of green and red tourmaline are highly prized gemstones. Quartzite is a hard metamorphic rock that originates from sandstone that has been heated and compressed, usually during tectonic compression. This sample is from San Diego, California, USA.