This is a fragment of a grinding stone. It is slightly concave on one face.
From Nubia (Sudan) Gebel Barkal, no. 514 foundation deposit. 1917: excavated by the Harvard University-Museum of Fine Arts Expedition; assigned to the MFA by the government of the Sudan.
Harvard University—Boston Museum of Fine Arts Expedition
Nubian, Napatan Period, reign of Aspelta, 593–568 B.C.