tourmaline -
a mineral that is a complex borosilicate and hydroxide of aluminum containing iron and magnesium and calcium and lithium and sodium; it is usually black but occurs in transparent colored forms that are used as gemstones
transparent gem a gemstone having the property of transmitting light without serious diffusion
mineral solid homogeneous inorganic substances occurring in nature having a definite chemical composition
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Wiktionary
Substantiv
tourmaline -
A complex black or dark coloured borosilicate mineral.
