[Mithadan]

My description of the shape and size of the Silmarils in Tales from Tol Eressea is from my own imagination. In fact, my personal vision of the jewels has changed over time. When I first read the Silmarillion, I imagined them to be unfaceted and round or oval. Later I viewed them as faceted oval cabochons (rounded on one side, slightly flattened on the other). As for size, I always viewed them as being similar to a small egg, about 80 carats or a bit more -- much larger than stones found typically in jewelry but smaller than, say, the Arkenstone (which I thought would be between the size of a baseball and a softball). This would be small enough for all three to be worn on Feanor's brows.

I have a vague recollection of JRRT comparing them to an egg somewhere, perhaps in Letters, but am not sure. I also seem to recall them referred to as cabochons, maybe in Lays of Beleriand, but they were definitely referred to as "globes", Lay of Leithian, line 1642. This description is found in the poetic version of the Oath of Feanor, which was later changed and the "globe" reference omitted. They are also referred to as "globes" in the abandoned poem, The Flight of the Noldoli from Valinor.