[ourmandave]

Melting points: sapphire - 3,722°F (2,050°C) niobium - 4,491°F (2,477°C)

[SomeHacker44]

Importantly, they likely have very different thermal conductivity and and specific heats. They probably also need to have reasonably similar thermal expansion coefficients so heating and cooling cycles do not cause them to strain and break.