Bone blocks can be found naturally in the nether, or crafted by putting four of them side by side like you would a crafting table. When mined, it drops bones. Mining it in the nether is a great way to get a lot of bone meal.
4 bones would be needed to be placed in a grid pattern to craft a bone block.
When you mine a bone block (which can be broke with your hand) it drops 2-3 bones. When you mine a bone block with silk touch you will get the bone block