refractory

noun
  1. 1

    A material or piece of material, such as a brick, that has a very high melting point.

adjective
  1. 1

    Obstinate and unruly; strongly opposed to something.

  2. 2

    Not affected by great heat.

  3. 3

    Difficult to treat.

  4. 4

    Incapable of registering a reaction or stimulus.

Source: https://en.wiktionary.org/wiki/refractory