Bones can only be destroyed with a Diamond Sword or possibly a Diamond Spear also. (TNT , C4, and grenades can naturally). Sometimes after the block has been sitting in that space for long enough time it will scream when being hit. (Same as the spider eggs but constant with every hit).
Recently The block has been found again but with a higher quality picture of the skull and crossbones with the name "Not Used 2" but it holds the same physics as the old block
While pressing 'Y' to prospect you will notice it says this. "A grisly find Be watchful, skeletons are here." Unfortunately, the block has no purpose yet, but it will most likely be used to spawn a skeleton mob if introduced in later updates.