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DIAMOND is the hardest material known in the nature. Hardness is defined as resistance of a material to scratching. It ranges between 1 and 10 on Mohs hardness scale. Where as 1 is the softest, 10 is the hardest one. Diamond has hardness of 10, i.e. H=10. Diamond is four times harder than the second hardest material in the nature, corundum from which sapphires and rubies are formed. Hardness of diamond can also be specified in other terms as 9000 kg/mm2. Whereas diamond is the hardest material known, graphite is one of the softest materials known. Because of its resistance to scratching, it is used in making of daily wear ornaments like engagement ring, wedding ring, necklace, ear rings etc.

Aggregated diamond nanorods (ADNRs) are a nanocrystalline type of diamond. It is even harder than diamond. It is even lesser compressible than diamond. ADRNs have a modulus of approximately 491 gigapascals (GPa) whereas diamonds have modulus of 442 GPa. ADNRs are also 0.3% denser than diamond. ADRNs are also called a hyperdiamond. ADNRs are formed by compressing fullerite powder at a high temperature. Fullerite is a form of carbon. Fullerites are the solid from of fullerenes (a carbon allotrope).

Specific gravity or density of diamond is 3.515 gm/cm3) . Diamond is the least compressible and stiffest substance known.

Australian diamonds are harder than the ones found in India and the USA. The hardest diamonds in world are found in Copeton & Bingara fields in New England area of New South Wales, Australia. They started to come in bulk in 1870s. These diamonds are tiny in size, perfect to semiperfect octahedra. Being harder than other diamonds, they can be used to cut and polish lesser hard diamonds.

Hardness

Hardness is a vector property. Hardness of diamond and many other substances varies with different crystal faces and also with direction of the faces. This has been established since long time by experience of diamond-cutters and gemologists. Crystals orientation to <111> direction are harder. However, if we assume that diamond is unbreakable, it is wrong. A diamond crystal is hard in one direction, it is soft in other direction. If it gets a stroke with sufficient force in a particular direction, it will break or crack.

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Hardness of diamonds depends on its purity and crystalline perfection. Flawless & pure diamonds are harder. Diamonds with defects in crystal structures are less hard. Diamonds with impurities also don't have perfect crystal structure, which results in less hardness.

The property of hardness makes diamonds an ideal material for cutting and grinding other materials. They are used in making cutting and grinding tools like drill machines, engraving tools, saws, rubbing tools, abrasive sand paper etc.

Though diamond is hardest material, it has some weakness as well. It has octahedral cleavage, i.e. there are 4 planes where weak forces exist in the valence bonds. So it will split easier, if force is applied in these directions as compared to other directions. When diamond is mounted in a jewellery piece, these weak planes are often positioned in such a way to keep them away from the direction from which they can get a stroke.