Prasiolite is the green variety of Quartz. The material is extremely rare in nature and is only found in a handful of deposits. The gem gets its name from the Greek prason meaning "leek" and lithos meaning "stone." Most Prasiolite on the market has been created by heat treating amethyst of a certain composition or has been synthesized. You will not find prasiolite in antique jewelry; the only known natural source that produces gem quality specimens is said to have been discovered in the 1950's in Brazil.

This 2009 paper speaks of irradiation causing some quartz to turn green.

Gemological information for Prasiolite
Color Green
Crystal Structure Trigonal
Refractive Index 1.544-1.553
Family Quartz
Durability Excellent
Hardness 7
Similar stones pale peridot, green beryl, green garnets
Treatments Heat treatment
Country of origin Brazil, Madagascar and Myanmar
Prasiolite care
Ultrasonic cleaning Not safe
Steam cleaning Not safe
Warm soapy water Safe
Chemical attact Usually safe
Light sensitivity Stable
Heat sensitivity Stable under normal conditions