Basalt is the most abundant igneous rock. The formation of these rocks from magma is very rapid – it can be a matter of hours or days. Magma is the ‘potential’ out of which many minerals and rocks will be formed. Magma is identical everywhere, which is why we have such a variety of minerals and basalt stones. This homogeneous matter, that consists of a mixture of substances (several minerals), is called rock. Most present day volcanoes erupt basalt material and are usually made of polycrystalline olivine and iron-magnesium silicate (olivine basalt) with a density that is 2.5 times the weight per volume of water.