Cleiophane

Cleiophane was an exceptionally beautiful pale green variety of sphalerite (a rock called zincblende or blackjack).

Description
This semi-precious stone yielded transparent gems of green "flash" in unusually large sizes. Faceted specimens 3 in (8 cm) across were known. However, cleiophane was soft and fragile, and such gems wore quickly. A typical stone had a base value of 50 gp.