1. Steal it from another player
From your dungeon, tap the "Attack" button, and then "Find a Match". You will then be able to attack a dungeon of a random player.
2. Dig it in the mine
From your dungeon, tap the "Attack" button. On the right side of the screen, there is a map with single player levels. You can unlock mines by playing in single player. Mines are represented by a trolley icon. Each mine will give you a gem once in a while.