BLACK TOURMALINE CRYSTAL
Black Tourmaline Crystal. Large, well formed Tourmaline Crystal! Beautiful and hard to come by this good. We were very lucky to come across such a beautiful piece. A superb specimen for the Minerals collector.
4.6cm x 4.4cm x 3cm, 100g.
Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH). The Tourmaline Group has a general formula of AX3Y6(BO3)3 Si6O18(O, OH, F)4. The A can be either calcium or sodium. The X can be either aluminum, iron, lithium or magnesium. The Y is usually aluminum, but can also be chromium or iron. Some potassium can be in the A position, some manganese can be in the X position and some vanadium can be found in the Y position, but these elements are usually not represented in the formulas of the tourmaline members.
There are many unique properties of tourmalines. First they are piezoelectric which means that when a crystal is heated or compressed (or vibrated) a different electrical charge will form at opposite ends of the crystal (an electrical potential). Conversely if an electrical potential is applied to the crystal, it will vibrate. Secondly they are pleochroic which means that the crystal will look darker in color when viewed down the long axis of the crystal than when viewed from the side. This property goes beyond the idea that the crystal is just thicker in that direction. Even equally dimensioned crystals will demonstrate this trait.