Olivine crystals are usually made of the elements silica, iron and magnesium, with more magnesium than iron.  Some forms of olivine also contain calcium as a substitute in the crystal structure for the iron or magnesium.  Olivine crystals have also been identified in meteorites, and in rocks from the Moon, Mars, and the dust from a comet.

Because they are stable at such high temperatures and pressures, olivine crystals break down to clay very easily when they are exposed to water on the Earth’s surface.  Due to this characteristic, they are seldom found in mineable quantities.  However, olivine sand is used to cast objects in aluminum.  Gem grade peridot crystals are found in Arizona and New Mexico, and also form the unusual green sand beaches on the Big Island of Hawaii. 

The lamproite lava and volcanic ash at the Crater are rich in olivine crystals.  If you examine a piece of the lava, you will see some dark areas ranging up to 1/8^th of an inch in size.  Olivine crystals are usually olive-green in color, and they are named for olives because of their color.  However, the dark background of the lamproite lava surrounding the olivine crystals obscures their green color.  If you see little yellow dots in the lamproite lava, those are the olivine crystals that have changed to clay.