The gem is that elusive Prismatic Shard PerfectionWhateFarm referred to. It's a rare drop so you're going to have to work at getting one, which usually means a lot of time in the Mines looking for the many things that have that rare chance of getting one.
Breaking loads of Iridium Nodes in Skull Cavern with a Luck Buff like Lucky Lunch/Pumpkin Soup/Ginger Ale/Spicy Eel to get you more nodes is your best bet. That Luck Buff will also get you more Treasure Rooms as you descend and the Treasure Chests in those rooms also have a decent chance of giving the Shard, so that's the best way to do it. Make (or trade Jade for) loads of Staircases and bring a lot of Bombs and your favorite healing food (Cheese is good) and just put down Staircases until at least Level 60 (Level 100+ is better) and start Bombing larger concentrations of Iridium Nodes in relatively safe levels (few or no flying Monsters). You'll get some Treasure Rooms along the way and maybe you'll get Lucky!
