Peridot is an unusual gemstone, an olivey-green that looks great against the silver setting. It's gem grade olivine, a very deep-forming mineral that is not often found in great shape by the time it has reached the surface. Peridotite is a rock with a heavy olivine content and is often found associated with sources of diamonds.
At higher levels within the crust, peridotite is often hydrated and altered to serpentinite. The original minerals are lost and new ones form to replace them.
Please note: at the moment we have tear-drop shapes, not square as pictured.