Tourmaline is a semi-precious crystal with a variable chemical structure, a complex silicate of aluminum and boron. Instead of aluminum atoms, atoms of sodium, calcium, magnesium, iron, lithium, etc. can be substituted in various proportions, causing variations in the physical properties of the crystal. Tourmaline is part of the minerals with the trigonal crystallization system. The chemical formula shows that hydroxyl ions (-OH) and fluoride ions (-F) can combine in different mixing ratios with metals, this mixture determining the color and hardness of the mineral (between 7 and 7.5).
Tourmaline colors are very varied, from blue, green, red, pink, brown, to black, sometimes a crystal can have several colors; in cross section some crystals appear as agate with concentric circular lines of different color. In tourmalines, the phenomenon of pleochroism can often be observed, i.e. the change of color depending on the direction of observation. The most common pleochroism is blue to black.
Another property of tourmaline crystals is piezoelectricity, which is manifested by the electric polarization of the crystal as a result of a mechanical action (pressure or torsion) applied on a certain axis of it. Also, most varieties of tourmaline also show the property of pyroelectricity, that is, the appearance of an electrical polarization following heating or cooling of the crystal. Tourmaline is the first material in which these two properties have been observed.
The term tourmaline arrived in the Romanian language from other European languages (in German Turmalin, French tourmaline). But at its origin is the Sinhalese word (the majority language spoken in Sri Lanka) turamali, meaning "ash-attracting stone", which reflects the pyroelectric properties of tourmaline.