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it seems to be petrified whatever it is.my first thought was root from tooth.
I'd say " your exactly right, the root portion of a tooth "
Asside from general apearance, the 'up & down ' split of the R side in the pic is typical of how teeth fracture.
Save it in a safe place for the future. MOST teeth are found without the roots and a bit of expoxy will make a complete restoration.