The molecular structure of a diamond makes it hard, as diamonds are comprised of carbon atoms linked closely together in a lattice structure. The atoms are linked tightly via covalent bonds wherein two atoms share an electron. A tetrahedral unit is composed of five carbon atoms, with one carbon atom sharing electrons with the other four. An extremely strong molecule is formed from the tetrahedral bonding of five carbon atoms.
In its natural form, carbon is not very hard. It is easy to crush when enough force is applied. However, carbon structure changes considerably when carbons are subjected to high pressure and heat. It transforms from something soft into something extremely hard. Carbon atoms compress and form a highly concentrated crystallized structure that gives the diamond exceptional hardness and valuable properties.
The crystallized form of carbon created from extreme pressure and heat leads to the creation of diamonds. Diamonds are incredibly hard because they have crystallized in a particular atomic shape that resulted from heat and pressure on earth at a depth of 140 to 150 kilometers. It takes a long time to create a diamond, and they are often brought to the Earth’s surface through a volcanic eruption. On the Mohs Hardness Scale, diamonds rank at 10, which is the hardest in the scale.