Crystal Hearts are objects found on any layer below the surface. When the player breaks a Crystal Heart using a pickaxe, drill, or explosives, it drops a Life Crystal which can be picked up. The Life Crystal can then be used to increase your maximum health by 1 heart, or 20 health. It also heals you by the same amount. Life Crystals cannot be used once the player has 400 health, at which point only Life Fruit can further increase the player's health to a maximum of 500.
While they can be hard to find, single-player mode worlds will actually contain dozens more Crystal Hearts than the player can use.