Translucent cross section of a tourmaline crystal. Tourmaline is a boron-silicate mineral, crystal system trigonal, and commonly forming crystals with an elongate prismatic habitat. It occurs in a number of colors, some of which are much sought after as semi-precious gemstones. This section illustrates the interior structure of the crystal, complete with the tri-axial cross and the six-sided circumference typical for the trigonal crystal symmetry. Actual diameter 80 mm