It's a cop-out I know, but I reckon any attempt to describe the silmarils is pointless, as I imagine them to be beautiful beyond our comprehension. I am inclined towards the idea that their shape is almost irrelevant; it was the light they contained (the light of the Trees) that characterized and defined the silmarils, a light so boundlessly captivating that the Noldor destroyed themselves trying to get the silmarils back.
Just my opinion; on the other hand they could look like large rubies
