Macor - is a machineable glass-ceramic, a white material that looks somewhat like porcelain. Macor has excellent thermal characteristics, acting as efficient insulation, and stable up to temperatures of 1000 ?C, with very little thermal expansion or outgassing. It can be machined into any desired shape using standard metalworking bits and tools.
Macor? is a registered trademark of Corning Incorporated. Using their trademarked name without the "?" is a violation of trademark law.
Just to help you look it up -here's their website. The first mention of the trademark in their citation uses the ?, and is not required in any following mention of their trademarked name. (sorry, I work with trademarks almost every day)