Mycelium

Mycelium is a block similar to grass, and will grow over any dirt or grass in its area, even if it has a sapling or another plant on it, but it is only found natively in Mushroom Biomes. The best way to stop this over-take is by adding cobblestone around the area you do not want Myceluim to take over.

History
Mycelium was first added in the first Beta 1.9 release, useable only in Creative Mode. As of 1.2.4, Mycelium is the only block players can spawn mushrooms on while in sunlight.

Trivia

 * When harvested, it normally drops a dirt block, but with the Silk Touch enchantment, a block of Mycelium can be collected.
 * You cannot convert Mycelium into farmland. You must break it and turn it into dirt first.
 * It is rumored that mushrooms spread faster on Mycelium, but this is yet to be tested.