In all likelihood, this oval, glass element was once mounted in a ring bezel. The perimeter of the glass is black while the center is a rough white.
From Meroe, Beg. N. 18, tomb of Queen Amanikhatashan. March 1921: excavated by the Harvard University-Museum of Fine Arts Expedition; assigned to the MFA by the government of Sudan.
Harvard University—Boston Museum of Fine Arts Expedition