Bone Block

Bone Blocks are blocks that are placed by the Bone Wand at the cost of 1 Bone per block.

In, mining them will drop Bones. There is no way to obtain the block as an item as of 1.4.

In, mining them will drop the Bone Block as an item. There is no way to convert them back into regular Bones. The block item cannot be used to craft Bone Block Walls or Bone furniture, which are instead made from raw Bones themselves, at a Bone Welder.

Trivia

 * On the, despite them being unobtainable in worlds and characters created in 1.4 or later, creating a world in 1.3.5 or earlier, collecting some bone blocks and then saving them in either the player's inventory or a storage device, it is possible to import them over to 1.4. However, despite them being researchable in Journey Mode, they can only be brought to those worlds with third-party programs. If a player does this, bone blocks can be freely duplicated in Journey Mode, however this is largely pointless aside from collectability, as bones and the Bone Wand can both be duplicated.
 * The Bone Block item appears to be a skull while the center block in a 3x3 square (as shown above) could possibly be bone marrow, which is what real life bones are made of.