Bones are dropped by the skeletal gorillas in the Ape Atoll Dungeon. Unlike regular bones, if these are left on the ground for too long, another skeletal gorilla will emerge from them and begin attacking any nearby players. If there are too many skeletal gorillas already spawned, they instead turn into "burning bones", which cannot be picked up. If buried, no gorilla will be spawned, and they will provide the player with 3 Prayer experience.
When you leave the dungeon with them in your inventory, a message will appear in the chatbox stating that "The skeleton bones you are carrying crumble to dust", and they turn into ashes.
Unlike other monkey bones, they cannot be turned into a greegree.
For an exhaustive list of all known sources for this item, see here