Crystal Shard

Crystal Shards are found in the Underground Hallow where they "grow" on Pearlstone Blocks, even when submerged in water or lava. Note, that they only appear in the cavern layer, and can be used as sign of real Underground Hallow, not just the Hallow in underground layer.

Unlike most other resources that grow/respawn, Crystal Shards can grow in any direction (i.e. on ceiling and walls). They cast a faint glow, like Demonite Ore, are quite common and sell for good money. They are also used in the creation of several useful items (in particular, Crystal Bullets and Greater Healing Potion), so it is wise to farm them when possible. They can also be placed for illumination just like a Torch, though not quite as luminous.