Diamonds form under very high pressure and temperature conditions within the Earth's mantle and are kinetically stable, meaning they remain in diamond form at the surface despite graphite being the more stable form of carbon at ambient pressure. Their unique structural and reflective properties contribute to their hardness and brilliance.
The conditions in which diamonds form can be described by understanding that diamonds are an allotrope of carbon that forms under very high pressures and temperatures deep within the Earth's mantle. Unlike graphite, which is stable under normal atmospheric conditions, diamonds are stable at these high pressures.
Due to graphite being the thermodynamically stable form of carbon at ambient pressure, diamonds should, in theory, convert to graphite; however, this process is kinetically hindered, making it extremely slow. As a result, once formed, diamonds are kinetically stable and can exist for extended periods even after being brought to the surface.
The kinetic stability of diamonds under ambient conditions coupled with their crystalline structure, where each carbon atom is bonded to four others in a tetrahedral configuration, contributes to the hardness and high melting point of diamonds. This structure also gives diamond the unique property of having a high index of refraction, leading to the intense sparkle diamonds are known for when light enters and is reflected within the crystal.