The tourmaline group includes a number of related species and varieties. They share a common crystal structure but have slightly different chemical compositions. Though tourmaline is a very popular gemstone, the different varieties distinguished by mineralogy are not well-known, even in the gemstone business. Both gem dealers and consumers are more familiar with a number of trade names for tourmaline.
The tourmaline varieties defined by mineralogy include: Buergerite and schorl, iron-containing tourmaline; dravite, which is magnesium tourmaline; elbaite, the most common tourmaline, containing lithium; liddicoatite, a type of calcium tourmaline; and tsilasite and uvite, which are manganese tourmaline.
The better-known trade names include rubellite, which is a ruby-red color; indicolite, referring to any shade of blue tourmaline; Paraiba, the neon blue-green tourmaline that is colored by traces of copper; verdelite, green tourmaline; watermelon, a tri-colored variety; and chrome tourmaline, intense green tourmaline colored by chromium.
Paraiba tourmaline has attracted a great deal of attention since its discovery in 1989. However, all blue tourmaline is quite rare, and high quality specimens are highly prized by collectors. Indicolite can range in color from light to deep blue. It is strongly pleochroic, meaning it shows different hues when viewed from different angles. Indicolite appears darker when viewed down the c-axis of the crystal and this must be taken into account when cutting. Poor cutting can result in a loss of transparency and brilliance, especially in darker specimens. Indicolite tourmaline is mainly found in small sizes and gems weighing over 1 carat are especially rare.
Although names like rubellite and indicolite are still used in the gem trade, they are becoming less frequent. It is becoming increasingly common to use color-specific names such as red tourmaline, blue tourmaline and so on, instead of the trade names. Even watermelon tourmaline is increasingly referred to as bi-color or tri-color tourmaline. This practice matches the test reports issued by well-known gemological labs who only use the trade names as secondary names, if they use them at all.
Indicolite or blue tourmaline is found in Brazil, Sri Lanka, the United States (Maine and California), Madagascar, Nigeria and Mozambique.
- Впервые опубликовано: January-25-2010
- Последняя редакция: August-16-2017
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