Name the hardest natureal substance known

State whether the following statements are true or false:Diamond (an allotrope of carbon) is the hardest natural substance known.

Name the hardest material.

Which one is the hardest natural substance among the following?

Compare the structure of the crystal of diamond graphite with special reference to the reason for diamond being the hardest natural substance while graphite one of the softest. Compare the electrical & thermal conductivity of the two crystalline allotropes of carbon.

Answer the following question in one sentence : Which is the hardest natural substance ?

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