Tourmaline
Tourmaline is derived from the Sinhalese word Turamali meaning 'stone with mixed colours' or 'mixed parcel' references on this are varied.. The colour range is certainly very wide and stones can possess an excellent colour saturation. Tourmaline is a very popular gemstone with good durability, the stone shown above is a deep pink tourmaline.
Tourmaline is a group of minerals, with complex chemical formulae, varieties include:
Rubellite: A red stone, similar in colour to ruby, some may have a violet tint. Stones can fetch high prices.
Pink tourmaline: A richly coloured pink variety; it is often difficult to distinguish strong pink stones from rubellite.
Green tourmaline: When termed 'chrome tourmaline' it is coloured by chromium, normal 'green tourmaline' is not.
Indicolite: A blue variety.
Dravite: A yellow to brown variety.
Schlor: A black variety.
Paraiba tourmaline: A rare blue stone coloured by copper, the colours may be intense; the most valuable tourmaline.
Cat's eye tourmaline: Included with long oriented parallel fibres; cut en cabochon to produce the chatoyant effect.
Colour change tourmaline: Coloured green in sunlight (daylight) and red in incandescent light (form a filament light bulb).
Source locations for tourmaline include: Afghanistan, Africa (Malawi, Namibia, Tanzania, Zaire, Zambia, Zimbabwe), Brazil (Minas Gerais, Paraiba), India, Europe (Italy, Switzerland), Madagascar, Myanmar, Pakistan, Sri Lanka, Russia and the USA (California, Maine).
Properties of the Gemstone and Additional Information
| Gemstone type: | Complex borosilicate of aluminium, magnesium, iron, calcium and alkali elements. |
| Crystal System: | Trigonal, occurring in long prisms that may have three, six or nine sides. The prism sides are often rounded. |
| Chemical Composition: | The chemical composition changes between the species of the tourmaline group. |
| Colour: | Tourmaline occurs in all colours, also parti-coloured (watermelon tourmaline) and multicoloured. Chrome tourmaline has a deeply staturated rich green colour, coloured by chromium. Green tourmalines not coloured by chromium have much less saturation. |
| Lustre: | Vitreous |
| Pleochroism: | Is strong in most stones showing a variation in colour depth, but absent in others. |
| Dispersion: | Low |
| Hardness: | 7 to 71/2 on the Mohs' scale |
| Toughness: | Good, but thermal shock 'sudden temperature change' may fracture a stone. |
| Cleavage: | Very uneven |
| Density in gm/cc: | 3.0 to 3.1 |
| Double refraction: | Yes |
| Refractive index: | 1.62 to 1.64 |
Common Treatments
Tourmaline may be heat-treated or irradiated to improve the colour. Prolonged exposure to heat or strong light may fade the stone.