Cordierite

Other names:

Iolite, Dichroite and Water Sapphire.

Appearance:

Transpartent or translucent; various shades of blue

Esoteric beliefs and associations...

Associated with the God Neptune (Poseidon)

Other useful information...

It's name comes from its violet colouring.

Physical properties...

Chemistry	Basic beryllium borate
System/Structure	Orthorhombic (prisms)
Refractive Index	1.530 - 1.559
Specific Gravity	2.58 - 2.66
Mohs' Hardness	7.0 - 7.5