Due to its crystal structure, silicon nitride is one of the toughest ceramic materials. Furthermore it has a very good thermoshock resistance and good tribological properties.
In contrast to other ceramics, boron nitride with the hexagonal crystal structure is soft and easy to machine. Special property: BN is not wetted by liquid metals such as aluminum.
Ruby is monocrystalline aluminum oxide where some of the aluminum atoms have been replaced by chromium atoms in a controlled way. This replacement is the cause of the typical ruby (red) colour.
Sapphire is monocrystalline aluminum oxide with controlled traces of other elements. As a transparent material, sapphire is suitable for optical applications.
Macor is the trade name of a machineable glass-ceramic, often used for electrical applications and at high temperatures.