This is a rectilinear fragment with 3 finely finished surfaces forming 2 corners. It is likely a fragment of a base or back pillar of an extremely fine object. The stone is highly polished dark brown, fine grained stone with white veins diagonal to the object.
From Nubia (Sudan), Kerma, K III 2(1)a. 1914: Excavated by the Harvard University–Boston Museum of Fine Arts Expedition; assigned to the MFA in the division of finds by the government of Sudan.
Harvard University—Boston Museum of Fine Arts Expedition