A diamond is a sort of transparent crystal of tetrahedrally bonded carbon atoms that crystallizes into the diamond lattice which is a variation of the face centered cubic structure. Diamonds have been adapted for various uses because of the material's exceptional physical characteristics. Best notable are its extreme hardness and thermal conductivity that is around 900-2,320 W•m-1•K-1, wide bandgap and high optical dispersion. Above 1,700 °C in vacuum or oxygen-free atmosphere, diamond converts to graphite; in air, transformation starts at ~700 °C. Diamonds occurring naturally have a density ranging from 3.15-3.53 g/cm3, with pure diamond close to 3.52 g/cm3. In spite of the hardness of diamonds, the chemical bonds that hold the carbon atoms in diamonds together are weaker than those that hold together the other form of pure carbon, graphite. The difference is that in diamonds, the bonds form an inflexible three-dimensional lattice whereas in graphite, the atoms are tightly bonded into sheets, which can slide easily over one another.

Diamond is the hardest natural material, where hardness is termed as resistance to scratching and is graded between 1 (softest) and 10 (hardest) using the Mohs scale of mineral hardness. Its hardness has been known since antiquity.

Hardness of the diamonds depends on its purity, crystalline perfection and orientation: hardness is higher for flawless, pure crystals oriented to the direction. Though it is possible to scratch some diamonds with other materials, such as boron nitride, the hardest diamonds can only be scratched by other diamonds. In particular, nanocrystalline diamond aggregates were measured to be harder than any single large crystal diamond. Those aggregates are generally produced by high-pressure high-temperature treatment of graphite or fullerite.

The hardness factor of diamond contributes to its suitability as a gemstone. Since it can only be scratched using other diamonds, it maintains its polish extremely well. Unlike many other gems, it is well-suited to wear daily because of its resistance to scratching, perhaps contributing to its popularity as the preferred gem in engagement or wedding rings, which are often worn every day.

The hardest natural diamonds originate from the Cope ton and Bingara fields, which is located in the New England area in New South Wales, Australia. These diamonds are usually small, perfect to semi perfect octahedral, and are used to polish other diamonds. Their hardness is associated with the crystal growth form, which is termed as single-stage crystal growth. Most other diamonds show more evidence of multiple growth stages which produce inclusions, flaws, and defect planes in the crystal lattice, all of which affect its hardness. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce harder diamonds that are used in hardness gauges.

Another factor related to hardness is a kind of mechanical property called toughness, which is a material's ability to resist breakage from forceful impact. The toughness of natural diamond is measured as 2.0 MPa•m1/2, and the critical stress intensity factor is 3.4 MN•m-3/2. Those values are better compared to other gemstones, but poor compared to most engineering materials. As with other material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond has a special cleavage plane and is therefore more fragile in some orientations than others. Diamond cutters use this attribute to cleave some stones before faceting.

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