Cleiophane

Clelophane is an exceptionally beautiful pale green variety of sphalerite (a rock called zincblende or blackjack). Clelophane yields transparent gems of green "flash" in unusually large sizes. Faceted specimens three inches (eight centimeters) across have been known. However, clelophane is soft and fragile and such gems wear quickly. A typical specimen has a base value of 50gp.