Tourmalines are a group of complex boron silicate minerals that come in a wide range of colors due to varying trace elements such as iron, manganese, lithium, and chromium. They have a Mohs hardness of 7 to 7.5, making them durable for jewelry. Tourmaline forms in igneous and metamorphic rocks, often found in pegmatites. Major sources include Brazil, Afghanistan, Madagascar, Mozambique, and the United States (California and Maine). Popular varieties include schorl (black tourmaline), verdelite (green tourmaline), rubellite (pink to red tourmaline), and indicolite (blue tourmaline).