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.

For farming Crystal Shards, one of the easiest methods is to find an Underground Hallow biome and create a vertical shaft of Pearlstone Blocks which alternates between two and four blocks wide. This allows for maneuverability while extending maximum potential growing area.

It should be noted it is possible to create a Crystal Shard farm by using Pearlstone in the Cavern Layer to create an artificial Underground Hallow. This will cause crystal shards to grow, much the same way as mushroom farms. It should be noted:
 * 1) You use a hammer to gather
 * 2) Properly "insulate" the farm using open areas or clay three blocks wide to prevent your farm from leeching out.