Diamond has a high refractive index due to the dense, tetrahedral arrangement of carbon atoms in its crystal structure and the strong covalent bonding between them. This structure causes a significant bending or slowing down of light as it passes through the diamond, resulting in a high refractive index. A high refractive index means that diamond has a strong ability to bend light, which contributes to its famous sparkling and brilliant appearance. When light enters a diamond, it is bent and reflected multiple times within the stone before it exits, dispersing into its spectral colours. This dispersion, combined with the stone's internal facets, creates the characteristic sparkle and fire that make diamonds particularly prized in jewellery. The high refractive index also means that diamonds can be cut in such a way as to maximize their brilliance, enhancing their visual appeal.

Graphite can indeed be transformed into diamond, but this process requires extreme conditions of high temperature and high pressure, similar to those found deep within the Earth where natural diamonds are formed. In laboratories, this transformation is achieved using two primary methods: High Pressure High Temperature (HPHT) and Chemical Vapor Deposition (CVD). In the HPHT process, graphite is subjected to temperatures above 1,400°C and pressures above 5 GPa. This environment forces the carbon atoms in graphite to rearrange into the tetrahedral structure of diamond. The CVD method involves breaking down gases like methane in a vacuum chamber to deposit carbon atoms onto a substrate, gradually building up a diamond structure. Both methods exploit the ability of carbon atoms to form different allotropes under different conditions and are used to create synthetic diamonds for industrial and gemstone purposes.

Graphite is used in some nuclear reactors as a neutron moderator. In a nuclear reactor, moderation is crucial for maintaining a controlled, sustained nuclear chain reaction. A moderator slows down fast neutrons, making them more likely to be captured by the fissile uranium-235, sustaining the chain reaction. Graphite is an excellent moderator due to its ability to slow down neutrons without absorbing them. Its layered structure, where weak Van der Waals forces allow layers to slide easily, also contributes to its stability under the high temperatures and intense radiation found in reactors. Furthermore, graphite's high melting point and chemical inertness make it suitable for the harsh environment inside a reactor. These properties, combined with its ability to conduct heat, help in maintaining the reactor's temperature and safety.

The extraordinary hardness of diamond is a direct result of its unique crystal structure. In a diamond, each carbon atom forms four strong covalent bonds with four other carbon atoms in a tetrahedral arrangement. This structure creates a rigid, three-dimensional lattice that extends throughout the entire crystal. The strength of these covalent bonds and the dense packing of the atoms contribute to diamond's unparalleled hardness. The tetrahedral bonding ensures that to break or deform a diamond, a significant amount of energy must be applied to disrupt the strong covalent bonds. This structure also contributes to the uniformity in hardness, as it is equally hard in all directions, a property known as isotropic hardness. Therefore, it is the density and strength of these covalent bonds, along with the geometric arrangement of the atoms, that make diamond the hardest known natural substance.

The mining of graphite and diamonds has significant environmental impacts, including habitat destruction, soil erosion, and pollution. Graphite mining, often open-pit, can lead to the clearing of land, affecting local ecosystems and wildlife. Additionally, the fine particulate matter from graphite can contaminate air and water sources, posing health risks to nearby communities. Diamond mining, especially in open-pit and alluvial mining, can lead to extensive ecological damage, including the removal of large amounts of soil and the disruption of riverbeds.

To address these issues, various measures are being implemented. In graphite mining, efforts include the rehabilitation of mined land, water purification systems to prevent contamination, and dust control measures. For diamond mining, the Kimberley Process was established to prevent "conflict diamonds" from entering the market, aiming to reduce the funding of armed conflict. Additionally, some diamond mines have adopted more sustainable practices, such as restoring ecosystems after mining operations and using more environmentally friendly mining techniques. Despite these efforts, the need for more stringent and widespread environmental regulations and rehabilitation practices remains a pressing concern in the mining industry.