|Peridot is not a "constant formula" gem, most stones lie somewhere between the chemistry of the two end members of the series. It is often found in volcanic bombs as very fine-grained centers. It is also found in some metallic meteorites and thus is produced outside the confines of the earth.
Peridot gems tend to the smaller sizes and large stones (over 100 ct) are very rare. The color can be from a light green, to middle green, with some stones sowing yellow or brown tones. The best quality peridot is pure green without any brown overtones. Higher iron content produces darker green stones, but too much iron turns the stones brownish or mirky.
Although not a rare mineral, it is rare in larger gemstone sizes and thus maintains a good price in stones over a few ct. Not an overly popular stone, it comes in and out of fashion with the release of new fabric colors each year. It does not have much of a continuous following.