Crystal Shard

Crystal Shards are found in the Underground Hallow where they "grow" on Pearlstone Blocks. Unlike most other resources that grow/respawn, Crystal Shard can grow in any direction (i.e. on ceiling and walls). The will also spawn in water or lava. The cystals are iridescent: they will glow in the presence of a light source. Otherwise, they are dark outside the range of the light source. (Note that their glow will NOT illuminate the area around them. Only the crystal itself will be visible, similar to how Spelunker Potions behave.) They can be broken with a Hammer.

They are quite common and sell for good money. They are also used in the creation of several useful items (in particular, Crystal Bullets), so it is wise to farm them when possible. They can be placed anywhere a Torch can in addition to underwater or even under lava.