White sapphire

Advantages:

• very hard material, very high heat conductivity
• made out of the extremely common element aluminum (more common than carbon)
• thermodynamically stable not just metastable thus very heat resistant

Disadvantage:
Crystal structure of less high symmetry than other interesting base materials.
Maybe look at metastable polymorphs at the eventual cost of somewhat less heat resistance?

Note: The page uses leukosapphire instead of just sapphire because
just sapphire is often associated with a blue to black variant where the colour is caused by metal impurities.
A base material for gemstone metamaterial technology would be perfectly impurity free and colorless clear though.
Like a leukosapphire just better.

Related

• Moissanite is also an extremely heat resistant base material.
• Diamond is much less heat and oxidation resistant.
• Both diamond and moissanite have higher crystal structure symmetry than leukosapphire