Diamonds may be the most popular gemstone in the world; however, to be a gemmologist, one must also love the many varieties of colour gemstones.
In that regard, surely tourmaline, known for its array of beautiful colours - sometimes with multiple colours in the one stone -would be a firm favourite. However, tourmaline is also one of the most complicated groups of minerals to study.
For starters, it is a very large family. While its most popular members are the red to pink stones (rubellite), blue stones (indicolite), green stones (verdelite and chrome tourmaline), and neon blue, green, and teal stones (Paraiba), the scientific sorting and nomenclature of the stones is much more complex.
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THE FACTS |
| Family: | Tourmaline |
| Hardness: | 7.75 |
| Pleochroism: | Strong; differential absorption |
| Main colours: | Pink-red, blue, green, vibrant green, teal and blue, multi-coloured |
| Main sources: | Brazil, Madagascar, Tanzania, Mozambique, Namibia |
| Acceptable treatment: | Heat |
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The tourmaline umbrella consists of more than 50 different mineral varieties, which are classified under three primary groups (alkali, calcic, vacancy) based on their chemical composition. Now, it gets tricky.
Let’s say you know a mineral family is proving difficult when the simplified version of its chemical formula resembles alphabet soup and instead is best expressed with the words “complex boro silicate” - which seems to be the scientific way of saying “it’s complicated!”
While most of the tourmalines we use in jewellery belong to either the elbaite (alkali) or fluor-liddicoatite (calcic) varieties, the truth is that it is impossible to identify the variety of tourmaline with standard gemstone testing.
In gemmology, we can separate gemstones into groups by their constant physical properties. For example, we know that members of the crystalline quartz family, such as amethyst and citrine, will have a refractive index (RI) of 1.54-1.55 and a specific gravity (SG) measure of 2.65; sapphires will have an RI of 1.76-1.77 and an SG of approximately 3.95.
Indeed, most gemstones have identifiable constant physical properties, despite being found around the world. The constants vary and overlap when it comes to tourmaline.
Pinning the different gemstones into distinct groups and varieties is so tricky that calling them all ‘tourmaline’ is easier. Without access to specialised equipment, we identify them by their colours, for example, ‘watermelon tourmaline’, a multi-coloured gem with a pink core and a green outer layer.
With that said, even that has proven fraught. As exciting as it must have been to discover the copper-bearing tourmaline in Paraiba state, in Brazil, in the 1980s, it added yet another gemstone and chemical variant to the family.
When similar vibrant neon blue, green and teal colour tourmalines, also caused by traces of copper and manganese, were discovered in the early 2000s in African countries, it prompted discussion over whether they could also be called 'Paraiba tourmaline'.
Technically, the gemstones from Brazil are ‘Paraiba tourmaline’, and the gemstones with the same composition; however, mined from African deposits are ‘Paraiba-type tourmaline’. For most purposes, the naming convention is to simply call them all Paraiba tourmaline, with a country of origin listed on reports.
In some ways, this confusion surrounding tourmaline continues its long history. For centuries, tourmaline was often mistaken for other gemstones, with the green stones mistaken for emeralds. It wasn’t until 1707 that the name ‘tourmaline’ was recorded in Western literature.
Gemmology has come a long way since then, but perhaps it is fitting that the tourmaline group of gemstones still confounds scientists today.
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