Bone has many symbololic significations.

Bone is a stone in the sense that it is both mineral and animal.

For practicing Buddhists, as well as others, it immediately makes one think of death and impermance, that we should always keep our minds from being swallowed up by distractons.

Flesh and bone, in the anturra Yoga tantra, are considered to be the basest state or the most concrete of white and red energies.
Red energy is feminine and is connected to warmth, desire and wisdom.
White energy is masculine and is connected to freshness, anger and moderation.

Bone is most often used in wrathful practices.

It can also represent the connection we have with animals and other human beings and in this sense is a symbol of compassion.

This bone is buffalo bone and is natural.