What are Astragalus Bones? They are bones that are located in the rear legs of four legged animals (in this case, deer) located at the heel. They are part of an articulating hind limb at the joint that connects the calcaneum and tibia which we see as the rear most angular joint of the hind leg.
These bones have been traditionally used for a socket in the bow and spindle fire method, and are very durable. They are prized for their authentic part of traditional primitive fire kits, as well as great performance. Sockets are not so easy to come by naturally. You can find some rocks, shells, etc., and you can just use another block of hard wood with a hollowed pivot point, but the socket depth and durability of these materials may make it difficult or limited in their use.
Once removed from the leg, they can be boiled to harden and any connective tissue is more easily removed from the bone.