By Yaĝé Enigmus
When pure, elbaite tourmaline is colourless. The colourless variety of tourmaline is known by the name “achroite”, which is derived from the Greek word “achroos”, meaning “no colour”. Tourmaline that is truly colourless is extremely scarce and is considered to be one of the rarest tourmaline gemstones.
An achroite elbaite from Paprok, Nuristan, Afghanistan. Image:
Rubellite Tourmaline (and Pink Tourmaline)
The presence of trivalent manganese (Mn3+) impurities in elbaite’s structure lends red and pink hues to the stone. When the saturation of manganese is sufficient enough to produce the perception of red or magenta, the name “rubellite” is often used. Rubellite’s name is a direct reference to similarities between the colour of red tourmaline and the colour of rubies. In fact, rubellite tourmaline was frequently mistaken for ruby in the ancient world since colour was used as the most important identifying factor for gemstones at the time. When not as highly saturated, these same manganese impurities in elbaite produce the perception of pink tones; this form of elbaite is simply known as “pink tourmaline”.
An elbaite tourmaline crystal from the Tourmaline Queen Mine, San Diego County, California, USA showing both zones of pink tourmaline and rubellite tourmaline. Image: SLR/ Mindat
Iron impurities may contribute to the colour of elbaite in a number of ways, but on its own iron produces blue colour centres. Ferric iron (Fe2+) impurities produce certain wavelengths of blue-indigo, and other wavelengths are produced by an iron to iron charge transfer (Fe2+-Fe3+). The blue of elbaite tourmaline may be a bright watery blue, often called “lagoon blue”, or it may be a dark indigo-blue, often referred to as “ink blue”. Tourmaline stones of blue colour are known as “indicolite”, a name meant to reference the colour indigo; in the strictest sense only pure blue or pure indigo stones are considered to be indicolite tourmaline, but such stones are rare and consequently this name may sometimes be used to describe stones with a greenish blue colour.An indicolite tourmaline crystal in quartz from the Santa Rosa Mine, Minas Gerais, Brazil showing zones of both dark “lagoon blue” and “ink blue”. Image: Fabre Minerals/ Mindat
Iron may also produce the perception of green colours in tourmaline. When present alongside quadrivalent titanium (Ti4+), the blue colour caused by ferric iron (Fe2+) impurities may be mixed with earth tones produced by an iron to titanium charge transfer (Fe2+-Ti4+) that some iron takes part in, resulting in the perception of green colouration. Green tourmaline can display a number of green hues depending on the ratio of impurities present and is known as “verdelite”, a name derived from the Spanish word for green “verde”, however this name is best used to describe stones of a pure green or forest green colour.
A verdelite tourmaline crystal from Minas Gerais, Brazil. Image: Antonia Borelli/ Mindat
The presence of divalent manganese (Mn2+) can produce a bright yellow colour in elbaite, but this form of manganese often does not produce visible colour on its own. Typically these impurities must be in the present alongside trace amounts of quadrivalent titanium (Ti4+), allowing some of the manganese to take part in a manganese to titanium charge transfer (Mn2+-Ti4+) which is thought to modulate the chromophoric properties of the remaining divalent manganese (Mn2+) . Yellow tourmaline which is particularly pure of colour is known as “canary tourmaline” in reference to the bright colour of canaries, but these stones are rather rare.
A yellow elbaite tourmaline crystal on matrix from Grotta d’Oggi, Livorno Province, Tuscany, Italy. Image: Matteo Chinellato/Mindat
These are merely the purer hues exhibited by elbaite. Learn more about the other colours of elbaite tourmaline in the next installment of this series.
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© Yaĝé Enigmus