How does carbon change into a diamond?

Hint: The carbon is a p-block element placed in the 14th group and the second period of the periodic table. The atomic number of the carbon is six and its electronic configuration is $\left[ {{\text{He}}} \right]{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{4}}}$. Diamond, Graphite, Graphene, fullerenes, carbon nanotubes are the allotropes of the carbon. All these are different forms of carbon and possess different physical and chemical properties mainly depending on their structures. The forces of attraction between the atoms of the allotropes diced its nature. The stronger the force, the more rigid is the compound while the weaker is the force, softer is the compound.

Complete step by step answer:
Diamond is an allotrope of carbon that exhibits cubic crystal structure with each carbon is tetrahedrally linked to four other carbon atoms and forms the 3D structure. These carbons are linked to each other by the strong force of attraction hence, the diamond is rigid.

Diamond is formed in the depth of the earth due to high pressure and temperature. This process required millions of years because that large three-dimensional structure of the tetrahedrally linked carbon atoms is formed known as diamond. This is the natural way by which carbon gets converted into a diamond.

In artificial methods are the High temperature and High-pressure method, and chemical vapor deposition technique. In both these ways that by natural way or by the artificial way there is phase transition that takes place when the diamond is formed from the carbon. In this way, carbon is converted into a diamond.

Note: Allotropes are the forms of the same element having different chemical and physical properties. Elements like carbon, sulfur, phosphorus, oxygen exhibit the allotropes.
The differences in the structure of the allotropes give rise to the difference in their properties of the allotropes even if they are made up of the same element.