Crumble Horn is an item added by Twilight Forest that is obtained from the Quest Ram after giving it all kinds of wool (the basic colors from Vanilla Minecraft) and, as the name implies, crumbles stone.
When used on a block, stone will be turned into cobblestone. If used again on cobblestone, it will then turn into gravel. After gravel it will not degrade into any other blocks. It can also be used on stone bricks, turning them into cracked stone bricks. Each time it is used one durability is removed (out of 1024).
When used in underground, it can be used as a great source of gravel or flint, making getting flint easier.