They are black hard volcanic igneous rocks that contain less than 53% silica. (SiO2) Basalt rock is a superficial fire rock whose texture (glass) or microscopic flour is because its crystals solidified near the surface of the earth. This causes the ions not to give the opportunity to cluster around the crystallization center, so its texture is very fine. Basalt is a very heavy and hard rock when touched. It is rough and very difficult to separate.
