This is a fragment of a proper left upper arm and chest of a medium sized statue. The stone is black granodiorite, the break areas look almost melted (often with large pink feldspar veins or inclusions). GD4.
From Nubia (Sudan), Kerma, K III 4(1). 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