I think its ceramic that is metalized and then coated to be nonconductive on the outside and the ceramic is in direct contact with the diode body and it feels like a decent heat sink. I have a piece of aluminum nitride here and it feels similar in terms of how cool it is when you touch it... like stone
It was a little confusing because when the diodes are in their baggy and you press them on your skin they feel cool like metal. I have the aluminum nitride plate in a similar ziplock bag (just came in), and it also feels cool like that when you press it to your skin, like metal. Plastic feels substantially different and much less cooling.
Are those glass diodes in a tiny glass tube, or are they enveloped in glass? As far as I can tell this diode is enveloped in the ceramic. The lead is thick copper. I sheared it in such a way that I think the tiny block of material I see at the shear junction is the diode,
The diode is about 1/4 of the surface area of the copper slug that terminates into the copper wire, so the construction of the conductive portion is
thin conductive lead, I think copper, that goes into a thicker copper slug. There are two slugs that sandwich the diode between them. This is all surrounded by ceramic. So they sink heat into the ceramic and into the lead. So the diode is on a surface 4x wider then itself and many times deeper, so those structures must wick the majority of the heat out. Since the lead is thin, I am not sure how much heat it is dissipating in comparison to the ceramic body. If it was in free air it seems that a round miniature heatsink would help it run cooler, but I don't know if the junction could be ran at a significantly higher power.
Interestingly since I don't think it requires a vacuum to run, its possible to chip away the ceramic and depending on how the diode is attached, solder it directly to a very large heatsink if it does not fall apart, and perhaps even put a drop of oil in there. Not that I would want to but I think that this might work.