Biotite gneiss is a variation of the commonly known metamorphic rock, gneiss. Most people will know the rock for being comprised of pink orthoclase feldspar with bands of white quartz and various micas, e.g. muscovite and biotite. Thus resulting in a pink, white and black banding pattern of sorts in the rock. However, these particular specimens contain mainly quartz, plagioclase feldspar (both being white minerals) and biotite mica, which is black. These result is a gneiss which exhibits a black and white banded appearance. They also show glistening bands of biotite mica on the exposed faces of the crystals. The banding, known as foliation are the alternating planar structures of the minerals . The specimens were collected by staff and come from Blairgowrie, Perthshire in Scotland and come supplied in various optional sizes in a card tray with an information label.