A non-metal X exists in two different forms Y and Z

A non-metal X exists in two different forms Y and Z. Y is hardest natural substance, whereas Z is a good conductor of electricity. Identify X, Y and Z.

A non-metal X is carbon ©. Carbon exists in two different forms called the allotropes of carbon. These allotropes are diamond and graphite.
Y is diamond because diamond is the hardest natural substance and Z is graphite which is a good conductor of electricity.