Toughness (3.05)
We’ve all heard the old saying “A diamond is forever.” Forever is an overstatement but since diamonds are extremely hard and do resist any kind of scratching, this would be a likely assumption. Diamonds will often break, given the right circumstances. How could an extremely hard substance break? It’s so hard that it’s brittle. Actually diamond’s toughness is rated “good” (not “excellent” like the gemstone jade). Remember that there is a significant difference between hardness and toughness. Hardness is simply the resistance to scratching and toughness is the resistance to breakage, to powdering or to flattening under repeated pounding.
The above image is courtesy of: r/HighQualityGemstones/
A fracture toughness scale is based on the amount of energy required to separate two surfaces or in this case diamond. Examples include 225,000 for nephrite, 120,000 for jadeite 5000-8000 for diamond and 600 for corundum (ruby and sapphire). The variance in the diamond toughness is dependent on the area of the crystal whereas a single grain area would be much less tough than where four grains converge. (The theoretical foundation of this scale originated with British aeronautical engineer A.A. Griffith in 1920).♦