Pink tourmaline polished on one side, originating from Madagascar.

Tourmaline (quartz) is a group of minerals, all cyclosilicates. The minerals in this group share a characteristic chemical formula: AX3Y6(BO3)3 Si6O18(O, OH, F)4. The A can contain calcium or sodium. The X can consist of aluminum, iron, lithium, or magnesium. The Y is usually made of aluminum, but it can also be made of chromium or iron. At position A, there can be some potassium, manganese can be found in X, and vanadium can be found in Y, but these elements are not often present in the formulas of the tourmaline group. Tourmaline occurs in pegmatites, metamorphites, magmatites, and alluvial deposits. Although tourmaline was already known in antiquity in the Mediterranean region, it was only introduced to Western Europe by the Dutch from Sri Lanka in 1703. They named the new gemstone with a Sinhalese word "Turmali," which means "stone with mixed colors." Traditionally, rubellites were used as gemstones; they were used by artists as talismans because they were believed to enhance the creativity of artists. Tourmalines have certain unique properties. They are piezoelectric, which means that when a crystal is heated or compressed, different electrical charges are formed at both ends of the crystal. This creates an electric potential. When an external electric potential is applied to the crystal, it vibrates. The minerals are pleochroic, which means that the crystal appears darker in color along the longest axis of the crystal than when viewed perpendicular to that axis. The four most well-known and common tourmaline minerals have different colors and transparencies. Elbaite is transparent and a valuable gemstone. Schorl, which is rich in iron, is the most common mineral from the tourmaline group and is black and opaque. It is primarily formed in pegmatites, the extremely slowly cooling dikes of a magma. The two other tourmaline varieties that regularly occur are dravite and uvite. Dravite is usually brown and translucent and can grow very large. Uviet is translucent to opaque. Tourmaline is found in the sand fraction of Dutch Quaternary river sediments. It is, among other things, a characteristic component of Maas sands. In the heavy mineral analysis, as conducted in the Netherlands by the Geological Survey during the second half of the twentieth century, the mineral is classified in the so-called stable group.