Gems are valued for their beauty, rarity, and durability. Durability is a gemstone’s ability to withstand wear, heat, and chemicals. The concept of durability consists of three properties:
1. Hardness: How well a gemstone resists scratches and abrasion
2. Toughness: How well a gemstone resists breaking and chipping
3. Stability: How well a gemstone resists chemicals and thermal shock
Diamond is very stable and extremely hard, but only somewhat tough. The hardness is uneven along different directions of the crystal, and this has important implications both for the cutting process and the durability of diamonds cut in various shapes.
The hardness of a diamond is a result of its very compact crystal structure. “Diamond has an indentation or scratch hardness five times that of carbides. [Figure 1] illustrates the hardness of diamond compared to other materials according to both Moh’s [sic] and Knoop’s hardness scales. However, diamond is relatively brittle and will chip or fracture if it is not handled and protected against shock” (V.C. Venkatesh and S. Izman, Precision Engineering, 2007, p. 50). Diamond is the hardest known natural mineral on both the Mohs and Knoop scales, which characterize a mineral’s scratch resistance through the ability of a harder material to scratch a softer material.
Diamonds are very stable and invulnerable to virtually all acids. They can also withstand higher temperatures than most gemstones. Sudden extreme temperature changes can cause damage, however.
Diamonds can chip or fracture from hard impact, especially in areas where the carbon atoms are not tightly bonded. These areas, called cleavage planes, are the major source of damage to diamonds (figure 2). The toughness scale used in scientific literature is called a fracture toughness scale, but it is rarely mentioned in gemology. The toughness scale measures (in erg/cm2) the work required to separate two surfaces of a crystal along a certain crystallographic plane. Values for diamond generally range from 5,000 (along the cleavage planes) to over 8,000 (in the “hardest” direction). Compare those values to 225,000 for nephrite and 120,000 for jadeite, which are tougher, and 600 for corundum, which is not as tough.
There is no such thing as perfect toughness. Any gem will break, not just chip, if it is hit hard enough. Diamonds are very tough, but remember that if a cutter can purposely cleave (split) a diamond by giving it a sharp blow in the right direction, you can achieve the same thing if you hit it hard enough accidentally. You are more likely to damage a diamond at the girdle or at a point. Diamonds with exceptionally thin places on their girdles are very vulnerable. To avoid damage, you should only wear your diamond jewelry during activities where you are unlikely to hit them hard against solid objects.