This amulet of polished hematite represents a headrest. Such amulets were common in Late Period burials, where their magic was intended to lift the head of the deceased at the moment of resurrection.
From Giza: G 7540T, Room III [Excav. date] March 1929. Excavated by the Harvard University–Museum of Fine Arts Expedition; assigned to the MFA by the government of Egypt.
Harvard University—Boston Museum of Fine Arts Expedition