Peridot
August's vivid green birthstone — gem olivine coloured by its own iron content.
Also known as: Gem olivine, Chrysolite
beginner Mantle-derived; basalts and meteorites
What it is
Peridot is the gem variety of the mineral olivine, a magnesium iron silicate, (Mg,Fe)₂SiO₄. Unusually, its yellow-green to olive colour is an idiochromatic property of the stone's own iron rather than a trace impurity, so peridot is always green. It is 6.5 to 7 on the Mohs scale, adequate for jewellery but soft enough to scratch, and sensitive to acids.
Peridot forms deep in the mantle and reaches the surface in basalt lavas; it has even been found in some meteorites. Historic sources include Zabargad Island in the Red Sea (mined by the Egyptians), with modern material from Pakistan, Myanmar, China and Arizona. It is the traditional birthstone for August and is almost never treated.