Peridot

Peridot was the gem-quality version of olivine which occurred in volcanic basalts or with other quartz deposits.

Description
These gemstones were found in shades of light to dark olive green and were usually facet cut.

Powers
Peridots were frequently used as a material component for abjuration spells and in constructing magic items that offered protection against spells and enchantments. A typical specimen had a base value of 500 gp or 50 gp depending on market demand.