This is a rectilinear fragment with 2 worked surfaces and a small part of a third worked surface. They form an inside and an outside corner. It is possibly part of a back pillar. The stone is black serpentinite with blue-green mottled coloration on surface.
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