Diamond

Despite being the hardest natural substance on Earth, diamond is rated as 'good' for toughness due to its perfect cleavage in four directions. Intense impacts in specific directions during everyday wear can lead to fractures and breakage of the stone.

Diamond is highly stable under everyday conditions, with excellent resistance to light exposure, heat, and common household detergents. It remains unaffected by moisture or dry conditions, and contamination causes little to no change in its appearance.

Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond. The two most common allotropes of pure carbon are diamond and graphite. In graphite the bonds are sp orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. In diamond they are sp and the atoms form tetrahedra with each bound to four nearest neighbors. Tetrahedra are rigid, the bonds are strong, and of all known substances diamond has the greatest number of atoms per unit volume, which is why it is both the hardest and the least compressible. It also has a high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520 kg/m in pure diamond. In graphite, the bonds between nearest neighbors are even stronger but the bonds between planes are weak, so the planes can easily slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Of all known substances, it is the hardest and least compressible. It has the highest thermal conductivity and the highest sound velocity. It has low adhesion and friction, and its coefficient of thermal expansion is extremely low. Its optical transparency extends from the far infrared to the deep ultraviolet and it has high optical dispersion. It also has high electrical resistance. It is chemically inert, not reacting with most corrosive substances, and has excellent biological compatibility.

Diamonds in the mantle form through a metasomatic process where a C-O-H-N-S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C-O-H-N-S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (e.g., CO2 or CO3) or oxidation of a reduced phase such as methane. Using probes such as polarized light, photoluminescence and cathodoluminescence, a series of growth zones can be identified in diamonds. The characteristic pattern in diamonds from the lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher temperatures and pressures as well as the transport of the diamonds by convection.

The most common crystal structure of diamond is called diamond cubic. It is formed of unit cells (see the figure) stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell. Each side of the unit cell is 3.57 angstroms in length. A diamond cubic lattice can be thought of as two interpenetrating face-centered cubic lattices with one displaced by 1/4 of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point. Viewed from a <1 1 1> crystallographic direction, it is formed of layers stacked in a repeating ABCABC ... pattern. Diamonds can also form an ABAB ... structure, which is known as hexagonal diamond or lonsdaleite, but this is far less common and is formed under different conditions from cubic carbon.

Diamond is an extremely popular gemstone. However, because it is the hardest known naturally occurring mineral, it is commonly used to make extremely tough drills and blades used to penetrate other hard materials. Many dentists use diamond-tipped drills to treat cavities and polish teeth.

The diamond was thought to represent the sun in Ancient Egypt, whilst Hindus in Ancient India believed that they could attract lightning. Ancient Greeks and Romans thought diamonds were the tears of gods. Diamonds also symbolize faithfulness and commitment between two people in a relationship, which is why, in some cultures, a diamond ring is a symbol of engagement or intention to marry.

Diamonds are far from evenly distributed over the Earth. A rule of thumb known as Clifford's rule states that they are almost always found in kimberlites on the oldest part of cratons, the stable cores of continents with typical ages of 2.5 billion years or more. However, there are exceptions. The Argyle diamond mine in Australia, the largest producer of diamonds by weight in the world, is located in a mobile belt, also known as an orogenic belt, a weaker zone surrounding the central craton that has undergone compressional tectonics. Instead of kimberlite, the host rock is lamproite. Lamproites with diamonds that are not economically viable are also found in the United States, India and Australia. In addition, diamonds in the Wawa belt of the Superior province in Canada and microdiamonds in the island arc of Japan are found in a type of rock called lamprophyre. Kimberlites can be found in narrow (1 to 4 meters) dikes and sills, and in pipes with diameters that range from about 75 m to 1.5 km. Fresh rock is dark bluish green to greenish gray, but after exposure rapidly turns brown and crumbles. It is hybrid rock with a chaotic mixture of small minerals and rock fragments (clasts) up to the size of watermelons. They are a mixture of xenocrysts and xenoliths (minerals and rocks carried up from the lower crust and mantle), pieces of surface rock, altered minerals such as serpentine, and new minerals that crystallized during the eruption. The texture varies with depth. The composition forms a continuum with carbonatites, but the latter have too much oxygen for carbon to exist in a pure form. Instead, it is locked up in the mineral calcite (CaCO3). All three of the diamond-bearing rocks (kimberlite, lamproite and lamprophyre) lack certain minerals (melilite and kalsilite) that are incompatible with diamond formation. In kimberlite, olivine is large and conspicuous, while lamproite has Ti-phlogopite and lamprophyre has biotite and amphibole. They are all derived from magma types that erupt rapidly from small amounts of melt, are rich in volatiles and magnesium oxide, and are less oxidizing than more common mantle melts such as basalt. These characteristics allow the melts to carry diamonds to the surface before they dissolve.

Diamonds are extremely rare, with concentrations of at most parts per billion in source rock. Before the 20th century, most diamonds were found in alluvial deposits. Loose diamonds are also found along existing and ancient shorelines, where they tend to accumulate because of their size and density. Rarely, they have been found in glacial till (notably in Wisconsin and Indiana), but these deposits are not of commercial quality. These types of deposit were derived from localized igneous intrusions through weathering and transport by wind or water. Most diamonds come from the Earth's mantle, and most of this section discusses those diamonds. However, there are other sources. Some blocks of the crust, or terranes, have been buried deep enough as the crust thickened so they experienced ultra-high-pressure metamorphism. These have evenly distributed microdiamonds that show no sign of transport by magma. In addition, when meteorites strike the ground, the shock wave can produce high enough temperatures and pressures for microdiamonds and nanodiamonds to form. Impact-type microdiamonds can be used as an indicator of ancient impact craters. Popigai crater in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact. A common misconception is that diamonds are formed from highly compressed coal. Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first land plants. It is possible that diamonds can form from coal in subduction zones, but diamonds formed in this way are rare, and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal.

Diamond is believed to be one of the most powerful of stones. Said to offer access to Divine energy, the stone is often used to attract love, respect, and emotional growth while cleansing the aura from negativity. It is often worn on the Third Eye chakra to improve mental clarity and provide healing energy to the physical body.