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Articles from GEMSTONES - TOURMALINE (5 Articles)












Image courtesy Gia Edu | Red tourmaline (Left) | Pink tourmaline (Right)
Image courtesy Gia Edu | Red tourmaline (Left) | Pink tourmaline (Right)

Colour investigation: Tourmaline - Part I

Boasting an array of colour and colour combinations unrivalled by any gem species, one can find tourmaline in shades of almost every hue. Throughout history this gem was often confused with other minerals until tourmaline was identified as its own mineral species in the 1800s. STACEY LIM reports.

The name ‘toramalli’ (the Sinhalese word for “gems of mixed colours”) was originally used to describe these unknown coloured crystals.

The tourmalines are a group of mineral species called borosilicates, that is, they are silicates containing boron, but also a mix of other elements.

Consequently, they have a similar crystal structure but their varying chemical composition means they have different physical properties. In particular, the inclusion of other elements, such as chromium, iron and manganese, creates the variety of colour for which tourmaline is famous.

The major tourmaline species are elbaite, liddicoatite, dravite, uvite and schorl, but most gem tourmalines are elbaites, which contain sodium, lithium and aluminium. Coloured by traces of various other elements, elbaites can be blue, green, yellow, pink to red, colourless or zoned with a combination of colours.

"Nearly all tourmalines display differential absorption of light, where the hue and sometimes tone vary with the orientation of the crystal"

Traces of manganese, for example, are thought to produce reds and pinks, although pinks may also owe their hues to colour centres caused by either natural or laboratory-induced radiation.

Some varieties of tourmaline flaunt trade names such as ‘rubellite’, the name given to a range of red tourmalines, often tinged with orange, purple or brown. Some members of the trade argue that terms such as rubellite are archaic and varieties should be named after their colour and species alone.

Tourmaline typically forms as long prismatic crystals, allowing some unusual long slender cuts. Pink to red crystals often have more visible inclusions than other coloured varieties although clean crystals are not uncommon in sizes under 2 carats.

Nearly all tourmalines display differential absorption of light, where the hue and sometimes tone vary with the orientation of the crystal. Pink crystals appear darker pink in the direction of the optic axis (parallel to the length of the crystal), and lighter pink perpendicular to the optic axis.

This enables a degree of control over the depth of colour when fashioning a stone. Carefully controlled heating or irradiation can also alter colour, and while the results of both methods are rather uncertain, these treatments are still extensively used commercially, and are often very difficult to detect.

Known for showing multiple colours in a single crystal, ‘watermelon tourmaline’ is a particularly desirable parti-coloured variety. Looking exactly as it is named, these impressive crystals show pink in the centre, running down the crystal axis with a concentration of green around the outside. It is typically cut into slices to display this astonishing colour arrangement.

The warm hues of tourmaline display a wide range of colour intensity and tone and while rubellite is generally a more available coloured variety of tourmaline, those with a hue and depth of colour that approach the intensity of fine ruby and pink sapphire can attract higher prices.

A vibrant and impressive gemstone, tourmaline will continue to rival its precious counterparts with a seemingly endless array of colours.












ABOUT THE AUTHOR
Stacey Lim

Contributor • Registered GAA Gemmologist & Valuer


Stacey Lim FGAA BA Design, is a qualified gemmologist and gemmology teacher/assistant. She is a jewellery designer, marketing manager and passionate communicator on gemmology. For information on gemstones, visit: gem.org.au









Wednesday, 15 August, 2018 03:42pm
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