Known to the ancient Greeks as adamas ("tame'sles" or "bridleless") and sometimes called adamant, diamond is the hardest known naturally occurring material, scoring 10 on the old Mohs scale of mineral hardness. The material boron nitride, when in a form structurally identical to diamond (zincblende structure), is nearly as hard as diamond; a currently hypothetical material, beta carbon nitride, may also be as hard or harder in one form. Furthermore, it has been shown[2][3] that nanocrystalline diamond powder (sometimes called aggregated diamond nanorods) is tougher than diamond, i.e. performs better as abrasive material. In turn, using those new ultrahard materials for diamond testing, more accurate values are now known for diamond hardness. A (111) surface (normal to the largest diagonal of a cube) of type IIa diamond has a hardness value of 167 GPa (±6) when scratched with an nanodiamond tip, while the nanodiamond sample itself has a value of 310 GPa when tested with a nanodiamond tip [2]. However, the test only works properly with a tip made of harder material than the sample being tested. This means that the true value for nanodiamond is likely somewhat lower than 310 GPa.
Cubic diamonds have a perfect and easy octahedral cleavage, which means that they have four planes—directions following the faces of the octahedron where there are fewer bonds and therefore points of structural weakness—along which diamond can easily split (following a blunt impact), leaving smooth surfaces. Similarly, diamond's hardness is markedly directional: the hardest direction is the diagonal on the cube face, 100 times harder than the softest direction, which is the dodecahedral plane. The octahedral plane, followed by the axial directions on the cube plane, are intermediate between the two extremes. The diamond cutting process relies heavily on this directional hardness, as without it a diamond would be nearly impossible to fashion. Cleavage also plays a helpful role, especially in large stones where the cutter wishes to remove flawed material or to produce more than one stone from the same piece of rough.
Diamonds crystallize in the diamond cubic crystal system (space group Fd\bar{3}m) and consist of tetrahedrally, covalently bonded carbon atoms. A second form called lonsdaleite with hexagonal symmetry is also found, but it is extremely rare and forms in meteorites or in laboratory synthesis. The local environment of each atom is identical in the two structures. In terms of crystal habit, diamonds occur most often as euhedral (well-formed) or rounded octahedra and twinned, flattened octahedra known as macles (with a triangular outline). Other forms include dodecahedra and (rarely) cubes. There is some evidence that nitrogen impurities play an important role in the formation of euhedral crystals—the largest diamonds found, such as the Cullinan Diamond, have been shapeless or massive. These diamonds are type II and therefore contain little if any nitrogen (see Composition and color).
Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.
The faces of diamond octahedrons are highly lustrous due to their hardness; growth defects in the form of trigons or etch pits are often present on the faces, the former being triangular pits whose points are aligned with the faces of the octahedron. A diamond's fracture may be step-like, conchoidal (shell-like, similar to glass) or irregular. Diamonds which are nearly round due to the stepping tendency of octahedrons are commonly found coated in nyf, a gum-like skin; the combination of stepped faces, growth defects, and nyf produces a "scaly" or corrugated appearance, and such diamonds are termed crinkles. A significant number of diamonds crystallize anhedrally: that is, their forms are so distorted that few crystal faces are discernible. Some diamonds found in Brazil and the Democratic Republic of the Congo are cryptocrystalline and occur as opaque, darkly colored, spherical, radial masses of tiny crystals; these are known as ballas and are important to industry as they lack the cleavage planes of single-crystal diamond. Carbonado is a similar opaque microcrystalline form which occurs in shapeless masses. Like ballas diamond, carbonado lacks cleavage and its specific gravity varies widely, from 2.9–3.5. Bort diamonds, found in Brazil, Venezuela, and Guyana, are the most common type of industrial-grade diamond, also cryptocrystalline or otherwise poorly crystallized, but possessing cleavage, translucency, and lighter colors.
Due to its great hardness and strong molecular bonding, a cut diamond's facets and facet edges are observably the flattest and sharpest. A curious side effect of diamond's surface perfection is hydrophobia combined with lipophilia. The former property means a drop of water placed on a diamond will form a coherent droplet, whereas in most other minerals the water would spread out to cover the surface. Similarly, diamond is unusually lipophilic, meaning grease and oil readily collect on a diamond's surface. Whereas on other minerals oil would form coherent drops, on a diamond the oil would spread. This property is exploited in the use of so-called "grease pens," which apply a line of grease to the surface of a suspect diamond simulant. Diamond surfaces are hydrophobic when the surface carbon atoms terminate with a hydrogen atom and hydrophilic when the surface atoms terminate with an oxygen atom or hydroxyl radical. Treatment with gases or plasmas containing the appropriate gas, at temperatures of 450 C or higher, can change the surface property completely. Naturally occurring diamonds have a surface with less than a half monolayer coverage of oxygen, the balance being hydrogen and the behavior is moderately hydrophobic. This allows for separation from other minerals at the mine using the so-called "grease-belt".[4]
Diamond is so strong because of the shape the carbon atoms make. It's a very strong 3D shape, each carbon atom having four joined to it with covalent bonds.
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