The gem merchant runs the Ardougne Gem Stall in East Ardougne. He sells sapphires, emeralds, rubies, and diamonds.
His stall's default stock is 0. It changes when players buy and sell from his stall. It is possible to turn a profit by buying his gems and re-selling them on the Grand Exchange. However, since players rarely sell their gems to him, it is highly unlikely you will make a large profit.
The prices are:
He appears alongside other residents of East Ardougne in the Ratcatchers Mansion.