Minecraft wiki entry for Development_versions, associated text: Does not generate anywhere currently, is only available from the creative inventory.
Can be placed on the floor or ceiling, creating stalagmites or stalactites respectively.
Can be combined to form longer stalactites and stalagmites.
Stalagmites break if not attached to something below, and hurt entities which fall on top of them.
Enemies that fall on the stalagmites deal three times more fall damage
Stalactites fall if unsupported, and hurt entities they fall on.