Black tourmaline quality AB, tumbled from Aracuai/Brazil

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 aluminium, iron, lithium or magnesium. The Y is normally aluminium but can also be chromium or iron. At the position of A there can be some potassium, manganese can be in X and vanadium can be found in Y, but these elements do not often occur in the formulas of the tourmaline group. Tourmaline occurs in pegmatites, metamorphites, magmatites and alluvial deposits. Although tourmaline was known in the Mediterranean region since ancient times, it was not until 1703 that the Dutch introduced it to Western Europe from Sri Lanka. They named the new gemstone with a Sinhalese word, "Turmali", which means "stone of mixed colours". Rubellites were traditionally used as gemstones – they were used by artists as a talisman because they were said to increase the creative power 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 electrical potential. When an external electrical potential is applied to the crystal, it vibrates. The minerals are pleochroic, which means that the crystal is darker in colour 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 colours 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 extremely slowly cooling gangues of a magma. The two other tourmalines that occur regularly are dravite and uvite. Dravite is usually brown and translucent and can grow very large. Uvite is green, translucent to opaque. Tourmaline occurs in the sand fraction of Dutch Quaternary river sediments. It is a characteristic component of Meuse sands, among other things. In the heavy mineral analysis as it took place in the Netherlands at the National Geological Service during the second half of the twentieth century, the mineral is classified in the so-called stable group