Peridot

Peridot is gem-quality olivine, a silicate mineral. It is one of the few gemstones that occur in only one color, an olive green. The intensity and tint of the green depends on how much iron is contained in the crystal structure, so the color can vary from yellow to olive to brownish green. The most valued color is a dark olive green. Peridot has a hardness of 6.5-7 and is the birthstone for August.