This is a sample of biotite, a brown/black mica mineral, that commonly abundant in igneous or metamorphic rocks. It is characteristic of its perfect cleavage, vitreous lustre, and very soft hardness (softer than a nail). Under microscopic lenses, it is characteristic of having brown/reddish pleochroism and having birds-eye extinction. It can be used to relatively understand chemical compositions of rocks or determine low metamorphic grade.