Peridot is gem-quality olivine, which is a silicate mineral. It’s one of the few gemstones that occur only in one color, olive green. The intensity and tint of the green depends on the percentage of iron that’s contained in the crystal structure. Peridot can occur as pure green with no yellow or brown cast.
Peridot - pic courtesy of wikipedia
Olivine is often found in lava and meteorites. A peridot formed in volcanic activity tends to contain higher concentrations of lithium, nickel and zinc. Olivine is an abundant mineral, but gem-quality peridot is rare. Olivine tends to exist in a heavily weathered state, thus making it unsuitable for decorative use.
A cut and polished Peridot gemstone - pic courtesy of Wikipedia
The largest cut peridot olivine is 310 carats and is in the Smithsonian Museum in Washington, DC.
This is the birthstone for people born in the month of August.