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 Lichen is naturally generated on stone, andesite, diorite, granite, calcite, tuff and dripstone blocks underground.

Post-generation
bone meal will spread it on a random adjacent block if possible, otherwise bone meal will be wasted. It is possible to cover the sides and the top of an adjacent block by bonemealing one lichen, but not the back. Otherwise, Glow Lichen does not spread naturally.

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. Breaking them with an axe is the fastest, although they will not drop, even with Silk Touch.

Light Source
Glow Lichen emit a light level of 7.

Trivia

 * If the player removes all directional states using a debug stick, the block will be invisible but the light emission will remain, unless another block is placed there.
 * In 21w03a, placing a Glow Lichen inside a lava source causes the lava to convert to water, even in the Nether. This is most likely a bug in the code swapping the 2 liquids.
 * The flowing water can also be converted into water sources using kelp.