Diamond Toughness

Q: Somebody told me that diamonds are hard, but not tough. Is this really true? Do I have to be especially careful when setting them?

A: To be blunt, this statement is not true. Toughness is a measure of how easily a stone can be chipped or broken. It’s not hardness, since as we know, diamond is the hardest natural stone out there.

Diamond is not the toughest stone, but its toughness is rated as being quite good, better than most stones. It can be broken. Ask any diamond setter, who will tell you that he/she has broken or chipped any number of them over the years. But for the most part, it’s not a fragile stone either. Breaking or chipping a diamond is not all that easy. In some directions through the stone it’s damn hard to do. In others, it’s easier—diamond has four cleavage planes through which it can break or split more easily, if hit just right—but usually, when a diamond is broken or chipped, there are other circumstances involved, such as an unusually thin girdle (edge of the stone), or the broken part being the point of a marquise or pear shape, or the crown or pavilion angles of the stone being shallower than they should be. A well-cut diamond is actually pretty hard to break.

As I said, if cut correctly, and especially in some directions, diamond can be extremely resistant to certain types of forces. In scientific research into ultra-high pressures, often the apparatus used is as simple as two diamonds, cut with larger culets, arranged so that the culets are pressed towards each other in a hydraulic press. The small size of these surfaces means the pressures generated are greatly magnified, and if the correct geometry of diamond is used for these setups, pressures of multiple millions of pounds per square inch can be exerted in that tiny area between the culets of the stones without damaging them, while the transparency of the diamond allows the results of the experiment to be directly observed.

While a few stones are rated as tougher than diamond, it’s only a few. Jade, especially nephrite, is the prime example. Chrysoberyl is often very tough, as is spinel, especially the synthetics. But offhand, I’d say that’s about it. Ruby and sapphire have only about the same toughness as diamond, and the majority of other gems we commonly use are a good deal more fragile (less tough) than diamond. Some of them, like opal and most emeralds, are a great deal less tough. And of the stones rated better in toughness, the long term durability of these stones is, in practice, less than diamond’s, since their hardness is less, and they’ll get scratched up, even if they don’t get broken. And all of the stones used to simulate diamonds, including white sapphire, cubic zirconia, GGG, YAG, synthetic moissonite, and others, are less tough than diamond, in addition to being softer, though the S.M. is better than the other simulants in this regard.

by Peter W. Rowe M.F.A., G.G.