Glow Lichen

Glow Lichens are non-solid blocks, $1/16$ of a block in thickness that can be placed on all the sides of a solid block.

Natural generation
Glow Lichens are naturally generated in caves

Breaking
Glow Lichens can be destroyed with anything, but using shears is the only way to collect them. Efficiency on Shears has no effect when used with Glow Lichens.