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

 * When harvested, mycelium normally yields a dirt block, but the Silk Touch enchantment can be used to acquire a mycelium block. In Creative Mode, it can be equipped and placed like any other block.
 * Mycelium is only found naturally in Mushroom Land biomes, but it can spread anywhere or be pushed anywhere with a piston.
 * Mycelium spreads in exactly the same way as grass: A mycelium block can spread to any dirt block within one space above, one sideways, or three down. The mycelium needs light level 9+ above it and the dirt needs light level 4+ above it, and must not be covered by any light-impeding block i.e. water, ice, lava, slabs, stairs, farmland, or any opaque block.
 * Mycelium also dies under the same conditions as grass: when covered by one of the light-impeding blocks above and the light level at that block is below 4.
 * When covered in snow, the sides will use the same texture as snow-covered grass.

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.

Trivia
Mycelium looks very similar to The Corruption in Terraria, one of the many games which is inspired by Minecraft.