There are several manufacturers, here are a couple I have used, Cotronics extensively:
With our machinable ceramic materials you can easily machine on standard and conventional shop equipment. Available as Alumina Silicate and Glass Ceramics.
www.cotronics.com
Aremco's extensive line of machinable ceramics is designed to be easily cut, shaped, & finished using conventional machining techniques.
www.aremco.com
Use
sharp carbide tooling with a lot of clearance, it is somewhat brittle, as you would expect.
Most of these ceramics eat HSS for lunch.
But it produces good finishes & tight tolerances with a little care.
Cover machine ways & use a vacuum pickup while machining; don't need alumina ceramic dust grinding up your machines.
I have used it extensively to make electrical isolator bushings in high vac deposition systems.
(and electrical isolator bushings in home projects)
It is great for high heat applications, where a plastic bushing would melt or burn up.
Made a bushing to isolate the output post on an alternator 20+ years ago, & it is still in service.
There is another brand name that you have to machine 15-17% larger, then fire it in a kiln, to produce custom parts that look like glass when finished. (a real PITA)
Can't for the life of me remember the name for that one. Probably a good thing.
Pictured below are a couple of chunks from the stash, I have more, but it was buried & 115F outside where the shed is.
The rest are old parts that have been rattling around in a junk drawer for years.
The pointy parts were made to eliminate heat sinking from a fixture to the base plate of a vacuum system.
Fixture stood on the points of three of these.
As you can imagine, not much heat transfer from the fixture. thru the points, and into the base.
These are Macor, the best machinable ceramic I found.
The others were prototypes for a case/mount for a sensor chip, that was direct contact bonded to the center glass tube, micro wires welded from the chip to the pins, then a cover direct contact bonded to the case/mount.
Not a material you need often, but when you do.....