You can only get this after you save the Jeweler in Act 2. You can not missing him (you'll find him in the Water Logged passage, near the end of the act) in the quest Blood and Sand.
After you save him and complete his quest, he'll go back to town. You can then go back to town (he's on the left side) and speak to him, he'll allow you combine gems. which you should have enough of if you haven't sold any.
If you have 2 flawed gems of the same type, he'll initially allow you to combine them and create a normal gem (2 flawed emeralds become an emerald). This trophy will pop right after that (And an in game challenge will also be completed)