How about https://www.idealtruevalue.com/store/p/171727-3804-Cold-Ceramic-Stem-Fits-Many-Manufactures-No-Lead-Compliant-Produ.html More on reddit.com

They look like alumina or yttria stabilized zirconia (YSZ). Often alumina is yellow from nitrogen impurities during firing. Both are pretty mechanically robust and chemically stable. I've never looked into how they are made but I think they're probably pressure cast. Think slip cast but with air pressure so you get much thicker parts. It's more.complicated but that's the basic idea. For the smooth surfaces, since they're planar, 99percent sure they are machine polished. Probably with diamond. Took a quick look at existing parents and everyone's being suspiciously non-specific regarding the type of ceramic. Wish I could tell you more, most of this information is just ancillary to the work I do because I use different kinds of ceramic crucibles in my line of work. You could ask for information from a ceramics manufacturer perhaps. I have had good communications with Almath crucibles in the past and might recommend pinging their sales team for contact info with one of their engineers. They're a little more human than CoorsTek which is now too big to care about little people. I've never tried stalking LinkedIn for CoorsTek employees in an engineering dept but maybe you could weasle your way in that way? Best of luck More on reddit.com