Block of Bones (Bone Block) is a block you can craft with 9 bones, and obeys gravity. If dropped from higher than 3 blocks, it will break into 9 bones, same if you mine it. There are also Slabs, that can be made with 3 bones, does not obey gravity but can break into 3 bones if mined.
Found in...
Dungeons
Sometimes replaces Chests or Monster Spawner.
Strongholds
Near the lava for the End Portal.
Mineshaft / Jungle Temple
Ontop of Cobwebs.
Desert
Randomly slabs place instead of Dead Bushes.
Desert Temple
On the corners in the trap chest room.
The Nether
in Nether Forrtress