This is possibly part of the mid-section of a white crown from a royal statue. Traces of the back pillar remain. It is very damaged. The stone is grayish-green greywacke.
From Nubia (Sudan), Kerma, K X grave K 1099 debris. 1914: Excavated by the Harvard University–Boston Museum of Fine Arts Expedition; assigned to the MFA by the government of Sudan.
Harvard University—Boston Museum of Fine Arts Expedition