I'm intrigued. Having worked in the specialty ceramics industry for decades I have to ask: Is the ring silicon nitride? With the black color and high gloss edges it looks like it.
Hey, yeah, that was a really good catch. The non-oxides component does contain silicon nitride, but I don't think it's only composed of that, even besides the oxides part of it, I believe it's a mix of multiple oxides and non-oxides.