Mycelium is a rare variant of dirt that is found in the mushroom fields biome. It only generates naturally and cannot be found in any other biome. It has a particle effect that resembles tiny spores being constantly released from the surface.
A mycelium block (via screenshot)
Mycelium can only be found in the mushroom fields biome which make it exclusive to it. You can pick it up by using a tool enchanted with silk touch. If broken with a regular tool or hand it will drop as dirt instead. A shovel with silk touch is the fastest way to get mycelium.
Mycelium in a mushroom field biome (via screenshot)
Mycelium is primarily a decorative block and has no specific uses. You can place a mycelium block and it will slowly spread to dirt blocks that are close enough, effectively making a farm. Mycelium can be turned into dirt paths by using any type of shovel on it.
Mycelium is one of the rarest blocks in the game as it is only found in the mushroom biome. They have a particle effect which makes them stand out making them a popular decorative block.
A. Mycelium will grow on any dirt block in an area where other mycelium blocks exist, mycelium will spread to adjacent dirt blocks, as long as the light level is 9.
A. Mycelium can be picked up by using a tool with the silk touch enchantment.
A. When bone meal is used on mycelium some small mushrooms will grow on it.