A diamond sitting on anthracite. Both are types of carbon. Diamond is made of carbon atoms in a variation of the face-centred cubic lattice. It has very high covalent bonding, making diamond the hardest natural bulk material. Anthracite is a hard, lustrous variety of coal with a very high carbon content that releases very little tarry or other hydrocarbon vapours when burnt. It is classed as a smokeless fuel so can be used in cities for domestic heating.