Tourmaline

One of the specimens in my mineral collection consists of two tourmaline crystals on a matrix of white calcite.

The long crystal on top looks indigo blue in natural lighting, but it is, indeed, green if one shines a very bright light through it. To do this, I used the LED light in my cellphone, turned the specimen upside-down to put the larger crystal closer to the light, and was able to obtain the following photo:

Tourmaline is described as a boron silicate mineral because it typically contains (BO₃)₃, but a large variety of other elements combine in various ways to comprise any given tourmaline crystal. This makes tourmaline one of the most complex of all the mineral varieties. To illustrate the complexity of this mineral, here is the chemical formula:

_{3 6} (₆O₁₈) (BO₃)_{3 3}

I can't even begin to fathom how many permutations of the above formula are possible, but it is a rather large amount. However, there are 33 common and significant variations recognized by the International Mineralogical Association, listed in the Wikipedia article in the sources at the end of this post.

Different permuations can yield crystals of different colors, and some of those permutations are common-enough to have names of their own. For example, a tourmaline crystal that has the chemical permutation NaMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃OH is typically brown in color and is called "Dravite," whereas a tourmaline crystal that is composed of □(LiAl₂)Al₆Si₆O₁₈(BO₃)₃(OH)₃OH [where '□' indicates a vacancy in the 'X' position] is pink to light-tan in color and is called "Rossmanite." The most common tourmaline is the one called "Schorl" with a chemical formula NaFe²⁺₃Al₆Si₆O₁₈(BO₃)₃(OH)₃OH that makes it black in color.

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SOURCES
   ² Wikipedia: "Tourmaline"
   ¹ Minerals.net: "The Gemstone Tourmaline"
   ² Geology.com: Tourmaline: Earth’s most colorful mineral and gemstone

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