The large wedge-shaped crystal is staurolite. The surrounding rock is a mixture of fine-grained quartz and mica. You can see that the mica and quartz grains line up in parallel planes. This alignment, called foliation, formed in response to deformation of the rock. Inclusions of quartz and mica grains in the big staurolite crystal retain this foliation, showing that the staurolite grew after the rock was deformed.

Crossed polarizers with gypsum plate