Indeed, diamond is the hardest material in the world. Material hardness is a property determined by scratch resistance. A substance can only be scratched by something of equal or greater hardness. Therefore, only a diamond can scratch another diamond.
Diamond sits at the top of the Mohs scale of hardness at number 10 as the hardest material. The Mohs scale rates relative hardness among materials. But hardness is non-linear and it is calculated that diamond is many times harder than next hardest substance (corundum) at 9.
Does that mean that diamond is impervious to damage? The answer is a qualified “no”. Diamonds are amazingly durable and can be worn daily for generations without the slightest bit of damage. But they can also chip or even break under certain conditions.
Diamond is composed of pure carbon and the atomic bonding of the atoms makes it the hardest material. Extremely strong bonding makes diamond hardest among all other substances, but there is one direction where the bonds are not as strong and a diamond can be cleaved along this direction with impact. A diamond cutter takes direction into account and the result is that damage due to cleavage is rare in well-cut gem diamonds
The biggest risk to diamond’s durability is where a diamond comes to a point or at the perimeter edge (girdle). Points on marquises, pears and princess cuts are more susceptible to damage from impact. Like any other hard object, the thinner you cut it the more fragile it becomes. And points on fancy shaped diamonds need to be secured by the diamond setter with pressure which presents some risk. During wear these corners can also experience accidental impact and can be damaged. The only point on round stones is at the bottom which is not exposed, nor does it require a prong to be placed on it. Therefore round diamonds are far less susceptible to damage than many other shapes.
Girdle thickness is a consideration for durability. Diamonds with extremely thin girdles are more likely to experience chipping in setting or in daily wear than diamonds with greater thickness. This is one of the factors measured during laboratory certification and is taken into account in the final cut grade. For instance, a diamond with an extremely thin girdle cannot receive an Ideal Cut Grade from the AGS Laboratory.
Purity is also a consideration in diamond hardness and durability. Despite being the hardest substance diamonds usually contain inclusions. Fractures, feathers and other disruptions of the crystal lattice can lessen a diamonds ability to withstand scratching and damage, particularly if such features exist near points or on a very thin girdle.