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Tourmaline
Gemstone
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Tourmaline is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a wide variety of colors.
The name is derived from the... Wikipedia
Hardness (Mohs hardness scale): 7 – 7.5
Associated month: October
Luster: Vitreous
Mohs: 7.0-7.5
Birefringence: −0.018 to −0.040; typically about −0.020 but in dark stones it may reach −0.040
Category: Cyclosilicate