Is this really a petroglygh?
A petroglyph generally refers to "art work" on an immovable object so a cliff face, the walls of a cave etc... when they are portable such as this then it is sometimes called a fetish or an amulet. The object is broken and may have been an amulet or perhaps a "gorget" (a type of "brest plate" that indicates tribal status. May have been a charm used by a medicine man. It is an interesting object and one that would fall into the category of ground and incised stone "tools" and ornaments.
Tank good care of it, it was once a prized object by its former owner.