The chemical composition of a mineral can be expressed by a formula but said formula might not be exact. It simply means that this or that named mineral refers to definite limits. The variation between the two limits is called solid solution. For example, the glassy, brittle, translucent/transparent mineral known as olivine varies from forsterite which is a pure magnesium silicate, to fayalite, which is a pure iron silicate. Both are uncommon but the variations between the two are very common. There are also tephroite, roepperite, knebelite, and monticelllite. A gem variety of olivine is peridot, which is green, and there are green sand beaches in Hawaii.