Question

Coal is known as black diamond tell me why is such?

Answer 1

It is because coal is also a compound of carbon when it burns it only produces carbon dioxide that proves that it is made up of carbon.
But,diamond is also made up of carbon and considered as the hardest compound in the world.
Coal is not complex as diamond that's why its properties are different from diamond.
Not every coal considered as black diamond only the heavy and denser called anthracite considered as black diamond.

Answer 2

Carbonado

Carbonado, commonly known as black diamond, is one of the toughest forms of natural diamond. It is an impure form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon. It is found primarily in alluvial deposits in the Central African Republic and in Brazil.