olivine forsterite

divorth-deactivated20151002  asked:

Hey! So when i was on Fuerteventura, a Spanish island, there were places with small green stones scattered about the place. The guide said they are minerals from inside a volcano. Do you know what they are called, if they even have a name?

The green minerals are most likely olivine xenoliths, meaning they were formed at depth in the earth’s mantle and brought to the surface during volcanic eruptions. The volcanic rock sometimes weathers away, leaving the olivine by itself. Green olivine generally means that it is higher in magnesium (as opposed to iron) and is known as forsterite. Gem-quality olivine is known as peridot. 

Here are some examples of olivine, not from Fuerteventura, but from Lanzarote, also in the Canary Islands and formed by the Canary hotspot:

(1, 2, 3)