Tourmaline

Tourmaline (TUR-mə-leen) is a crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of colors.

Color Black to Bluish-Black to deep Brown, while magnesium-rich varieties are Brown to Yellow, and lithium-rich tourmalines are almost any color: Blue, Green, Red, Yellow, Pink, etc. Rarely, it is colorless.
Mohs scale hardness 7.0 - 7.5
Specific gravity 3.03 - 3.25
Refractive index 1.603 - 1.655