Tourmaline
Tourmalines occur in virtually any colour, most commonly black, green, pink and blue, and are often found as ‘parti-coloured’ gemstones, with two or three colours displayed in the same gem that is cut to preserve this effect. Some colours of tourmaline have their own name, such as the dark inky blue variety Indicolite, pink and green watermelon tourmaline, red rubellite or the electric green-blue highly valued Paraiba tourmaline, now a name for all tourmalines of this colour, whether they are found in the Paraiba state of Brazil, or in the deposits more recently found in Mozambique or Nigeria.
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Tourmalines are fashioned by faceting into a wide variety of shapes and cabochons, carvings and beads. They have a hardness of 7-7.5 on the Mohs scale and have good toughness, making them suitable for setting in jewellery, but may show wear and tear fairly easily if set in everyday worn rings. In general, these stones attract dust easily and will need frequent cleaning, but the use of ultrasonic and steam cleaners should be avoided as it may affect the colour and cause existing fracture to worsen.
Tourmalines are of the trigonal crystal system and rough crystals are commonly identified by the three-sided prismatic crystal with convex faces that make a rounded triangular cross section. Occasionally crystals can be six or nine sided. Crystals are usually long and are heavily striated down the length and colour zoning follows the trigonal symmetry. They have a vitreous lustre. Tourmaline crystals are found in pegmatites and other granitic rocks. Gem quality tourmaline is sourced mainly from Afghanistan, Pakistan, Russia, Brazil, Madagascar, Mozambique, Namibia, Tanzania and USA.
They are strongly pleochroic, meaning they can show more than one colour, sometimes even without the use of dichroscope. Because of this, the colour is usually stronger when seen down the length of the crystal than observed across the width and so paler stones are often cut with the table perpendicular to the length of the crystal and darker stones cut with the table parallel to it. Stones can also be chatoyant if they contain many parallel fibres or channels, and these are cut as cabochons to display the optical effect of a band of light hovering across the stone.
Some mineralogical names that are also used for tourmaline are;
- Elbaite, the most common gem quality variety which are colourless, green to blue, pink to red and parti-coloured
- Dravite, usually dark green and brown to black
- Schorl, that are black and rarely cut as gems
Colour in tourmalines is enhanced by heat treatment or irradiation. Dark stones like blue and green stones for example can be lightened by heating them. The colour in red stones can be made stronger with radiation. Most electric blue-green Paraiba tourmalines are originally pink to grey-blue copper rich tourmaline that has been heated. Yellow tourmaline is mostly heat treated to reduce the brown tones.
Inclusions in tourmalines are seen as irregular thread like cavities, healed fractures and flat films.
Testing tourmalines with a dichroscope will reveal their strong pleochroism. Dichroism can sometimes be seen as two different colours or two shades of the same colour. Pale stones like yellow tourmalines may be weakly pleochroic and it may be difficult to see in very dark stones due to the ordinary light ray being totally absorbed. Unless the stone is very dark, double refraction can be observed in some material just by using a 10x loupe.
Tourmalines are easily identified with the refractometer as they have a hi birefringence. Their refractive index is 1.62 to 1.65, with a DR of 0.014 to 0.021 and a uniaxial negative optic sign. They have a specific gravity of 3.0 to 3.1.
In lore, along with opal, tourmaline is one of the birthstones for October and the zodiac stone for Libra.
