The green jewel is a quest item used in Koschei's Troubles. Players require 90 Prayer to detect curses on the gem in the miniquest and doing so turns the gem green. Players may still interact with the gemstone to change its colour. If the player decides to try and cut the jewel, it will turn into a blue jewel. If a player attempts to crush the jewel using 90 Strength, the pressure will turn it into a red jewel.
When handed to Kharshai, he rewards you with a Viridian Balmung and an Antique lamp if he hasn't already given you one.