Your specimen is a piece of
biotite. Biotite is a brown mineral characterised by its flaky texture. It is a
common mineral in lots of rock, particularly granites, and is visible as small
brown flakes. It is also known as mica; biotite and muscovite (which is
colourless) are both types of mica. Biotite is sometimes used as a heat resistor
and large crystals have been found in Brazil and Greenland.