Mycelium

Mycelium is a block similar to grass, but it is only found natively in Mushroom Island Biomes. It can grow over dirt blocks that are in proximity in a similar way to grass. As of Beta 1.9 Pre-release 5, the player may select it from the Creative Mode inventory.

Behavior
Mycelium behaves in a number of ways:
 * When harvested, it will yield one dirt block, unless harvested using an enchanted "Silk Touch" tool then it will drop a Mycelium block (otherwise can only be acquired via inventory hacks, SMP commands from an op, or in Creative Mode).
 * It slowly spreads out to cover bare dirt blocks. Because Mycelium is only found in Mushroom Land biomes, providing it with a dirt path to grow into or using pistons are the only "legitimate" ways to grow it outside the Mushroom Land biome. As of Minecraft 1.0.0 it can be harvested by any tool with the Silk Touch enchantment.
 * It grows faster when light is near, unlike the habits of real fungi.
 * When covered in snow, the sides will use the same texture as snow-covered grass.
 * It is extremely sensitive to water. In the time it takes a Water source block to fully spread out and recede on flat land, 20-40% of Mycelium blocks under the water will have reverted to dirt.

Other behaviors:
 * Saplings and Flowers cannot be placed on Mycelium, but Mycelium can spread beneath them, which causes them to drop as items from the spot.
 * Mycelium is also the only block on which mushrooms can stay when an adjacent block is updated despite a light level that is too high for the Mushroom to remain.
 * Mushrooms spread faster on Mycelium than on other blocks.
 * Mycelium cannot be tilled with a Hoe.
 * The tops of Mycelium blocks have a particle effect that resembles tiny spores being released constantly from the surface. This effect persists even when the Mycelium block is covered in snow.
 * Mycelium and grass do not replace one another.
 * Generated caves and ravines in a Mushroom Biome will not remove Mycelium, often leaving a thin layer over a ravine or 'bars' of Mycelium covering a cave.
 * Only Mooshrooms can spawn on this block, no other mob (including hostile mobs) can.