Pyroxenes are so common that together they are considered rock-forming minerals. You can pronounce pyroxene "PEER-ix-ene" or "PIE-rox-ene," but the first tends to be American and the second British. Diopside has the formula CaMgSi2O6. The Si2O6 part signifies chains of silicon atoms bound together with oxygen atoms; the other atoms are arranged around the chains. The crystal form tends to be short prisms, and cleavage fragments have a nearly square cross section like this example. That is the main way to distinguish a pyroxene from the amphiboles (which have a lozenge-shaped cross section).
Other important pyroxenes include augite, the enstatite-hypersthene series and aegirine in igneous rocks; omphacite and jadeite in metamorphic rocks; and the lithium mineral spodumene in pegmatites. See them and more in the Pyroxene Gallery.