What is High Grade Ceramic ?

[Aladin]

What is high grade ceramic and carbon resistors are made from high grade ceramic why ?
why they are made from carbon ?
please explain reasons

[Astronuc]

High grade ceramic usually implies high chemical purity, low porosity and relatively free of flaws (microcracks), in this case for use as an insulator around the conducting carbon. While conducting, carbon is not as conducting as copper or aluminum.