Question

how are diamonds formed?

Answer 1

diamond is a allotropes of carbon which are always in the 3D structure
one carbon attached with other 4 carbons and this chain continuous to very large number

Answer 2

100 miles deep in the Earth, a layer separating our hospitable exterior from the molten core, known as the Mantle, is where diamonds were made.         Temperatures boiled above 2,000 degrees Fahrenheit and pressure exceeded 725,000 pounds per square inch.
                            
                 The extreme heat and pressure combined actually modified Graphite, a Crystalline Carbon, on the atomic level. This restructured Graphite’s molecular composition from a hexagonal sheet pattern into a triangular shape, resulting in diamond.