Basalt is an extrusive igneous or volcanic rock that has a low silica content, dark in colour, and is very rich in iron and magnesium. Basalt rock is mainly composed of pyroxene, olivine, and plagioclase and is the most common rock on the earth's surface. The texture of basalt rocks is coarsely porous as those holes are left by gas bubbles.http://www.hzxstone.com/slate/