Tourmaline is a complex silicate of boron with aluminium and several other metals. It usually occurs in well-developed hexagonal crystals. Hardness of tourmaline varies from 7 to 7.5 and the specific gravity ranges from 2.9 to 3.2. It is usually black in colour and its streak is uncoloured.

   Attractive coloured and transparent tourmalines are used as gem stones.

Corundum

    Corundum is an oxide of aluminium. It occurs as massive bodies and as hexagonal crystals. The characteristic property of this mineral is its hardness which is 9. Its specific gravity ranges from 3.9 to 4.1.

    Ruby is the red, transparent to translucent variety of corundum.

   Sapphire is the blue, transparent to translucent variety of corundum.

   Corundum is cheaply used as an abrasive. The transparent varieties are used as a gemstone.