This is a fragment of a pottery vessel of a vaguely (irregular) diamond shape similar to those sometimes used for making weights or pendants. This example has no suspension hole drilled through it. Possibly a model pendant/weight?
From Kerma, Cemetery S, deposit 33A?. 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 the Sudan. (Accession Date: September 19, 2006)
Harvard University–Boston Museum of Fine Arts Expedition