What is the meaning of a peridot?

Peridot is a green gem stone, that is actually gem quality Olivine - a magnesium iron silicate mineral with the general chemical formula - (Mg, Fe)2SiO4.

Olivine is actually a solid solution series with varying iron and magnesium content where:

100% magnesium, 0% iron = forsterite

0% magnesium, 100% iron = fayalite

Peridot contains more magnesium than iron and so is closer in composition to forsterite

The meaning and origin of the Word peridot seems to be somewhat confused. Some references state that it is derived from the Arabic Word faridat which means precious stone. It is also said that it may be derived from the French Word peritot which is used to describe olivine and apparently means unclear. This is thought to relate to the presence of inclusions within the mineral which normally make it appear cloudy

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