Cordierite

The gem grade variety of Cordierite, which can be mistaken as quartz in its massive form, (named after P.L.A. Cordier, French geologist) is known as Iolite.

Iolite is pleochoric, so from some angles appears a deep sapphire blue, from others it seems grey.

Because of its optical properties it was used by the seafaring Vikings as a navigational tool, to chart position, due to its colour and its use at sea it it also known as "water sapphire".