Hi,
In regard to the picture posted, I think that the shape of the two semi-egg shaped arches crossing over eachother is in fact a 2D representation of a 3D shape. The 3D shape being a semi-sphere or similar and outlined, so to speak, by 4 arms which are positioned equidistantly in a circle and arch up to a central point: thus forming a semi-sphere shape. If this is the case, what would such a 2D projection be called? And would anyone know how to identify the precise viewing angle required in order that an interpretation of 3D semi-sphere would appear as the 2D arches shape on the jewel?
Thanks in advance and please let me know if you have any questions. I did put this forward to a curator at the British Msueum but she wouldn't really consider it past zoomorphic identifications i.e. the arches representing boars. If I had some technical definitions to support the claim I think I may be able to make further progress.