Tourmaline is a semi precious gemstone that occurs in any colour and in a greater range than other common gemstones. It has a number of strange and quirky characteristics that this article will briefly explore, some of which can help to readily identify it.
Rated at 7 to 7 ½ on the Mohs scale of hardness, most tourmalines display strong pleochroism. Pleochroism is an optical phenomenon in which a substance appears to be different colours when observed at different angles, especially with polarized light. These colours can be different shades of the main body colour, or indeed two quite different colours.
Tourmaline also displays strong double refraction, a phenomenon where light is split into two rays, each travelling in a different direction. The effect is so strong that it can be observed with the aid of a 10x lens. If you look through a piece of tourmaline using a loupe then you can see the opposite facet edges of the stone appearing doubled. What you are actually seeing are two separate light rays making there appear to be two edges to each facet.
Due to the unusual crystal structure of the gemstone, tourmaline has a number of peculiar characteristics. It is pyroelectric, meaning that it develops opposite electrical charges at the two ends of a crystal when heated; it is piezoelectric, meaning that it develops a charge when stressed; and different crystal forms and sometimes colours are developed at either ends of tourmaline crystals. Colour zoning is another unusual attribute, seen most strongly in watermelon tourmaline, which resembles the green outer and pink inner of the well known fruit.
One famous form of tourmaline is the variety rubellite. Named from the Latin rubellus meaning reddish, rubellite ranges from red to shocking pink, although the most valuable specimens resemble ruby. A top rubellite will shine the same intense hue of red whether viewed in artificial light or daylight.
Another highly sought after variety was first found in the Brazilian state of Paraiba, although it is now mined for the most part in the neighbouring state of Rio Grande do Norte. Paraiba tourmaline is a brightly coloured turquoise to green colour. Most Paraiba tourmalines contain a large number of inclusions, and this is one of the reasons why a clean example can be worth a great deal of money.
If you have any tourmalines that you wish to have valued then please contact Simon Rufus of Grand Auctions, Folkestone, Kent.
- Posted by Simon Rufus