Black Kyanite

Black kyanite (Brazil)

One of the three polymorphs of Al2Sio5. Polymorphs are minerals which have the same chemical formula, but different internal atomic structure. This means that they have different crystal habits (the shape that the crystal forms) and very different physical properties. 

All three of these polymorphs are commonly found in metamorphic rocks, but because they form in different temperature and pressure regimes, they can be used to gauge the conditions that the metamorphic rock encountered during its creation. However all three minerals can be quite persistent and may not immediately change into other minerals as the rock undergoes further changes. The presence of any one of these polymorphs cannot be used as a sole indication of the history of  the metamorphic rock. It is better to look at assemblages -- groups of minerals which tend to occur together in a given set of metamorphic conditions.

Kyanite is typically blue, hence its name, but like many minerals will exhibit different colours in response to the inclusion of impurities. As well as the famous blue kyanite, you can also find green, orange, white and dark grey to black varieties. 

This photograph is of the item offered for sale.