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 may contain calcium or sodium. The X can be aluminum, iron, lithium or magnesium. The Y is normally aluminum, but can also be chrome or iron. There may be some potassium in the position of A, manganese may be in X, and vanadium may be found in Y, but these elements are uncommon in tourmaline group formulas. Tourmaline occurs in pegmatites, metamorphites, magmatites and alluvial deposits. Although tourmaline was already known in the Mediterranean in ancient times, it was only imported from Sri Lanka in 1703 by the Dutch in Western Europe. They named the new gemstone with a Sinhalese word "Turmali", which means "stone of mixed colors". Rubellites were traditionally used as gemstones - they were used by artists as a talisman, because they would increase the creativity of artists. Tourmalines have certain unique properties. They are piezoelectric, meaning that when a crystal is heated or compressed, different electrical charges are formed at both ends of the crystal. This creates an electric potential. If an external electrical potential is applied to the crystal, it vibrates. The minerals are pleochroic, meaning the crystal is darker in color when viewed along the longest axis of the crystal than when viewed perpendicular to that axis. The four best known and most 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 of the tourmaline group and is black and opaque. It is mainly formed in pegmatites, the extreme slowly cooling gangues of a magma. The two other tourmalines that occur regularly are dravite and uvite. Draviet is usually translucent brown and can grow very large. Uvite is green translucent to opaque. Tourmaline occurs in the sand fraction of Dutch Quaternary river sediments. It is, among other things, a characteristic component of Maaszanden. In the heavy mineral analysis that took place in the Netherlands at the National Geological Survey during the second half of the twentieth century, the mineral is classified in the so-called stable group.