Diamonds are hard, not tough. When someone says a diamond is the hardest natural mineral on the planet, they would be correct. Despite this, there is indeed a difference between a mineral being "hard" and being "tough". Hardness, measured on what we call the Moh's scale, represents a material's resistance to being scratched or scathed. If you rub two parallel surfaces together and one is harder than the other, the softer one gets scratched. Toughness on the other hand, refers to a material's resistance to splitting and breaking.
What most people don't know or remember- is that diamond is not the king of toughness. It actually has 4 directions of perfect cleavage. This means that the species possesses four directional planes of atomic weakness, that would make it prone to splitting into smaller pieces. In the early days of diamond cutting, manufacturers would make use of this property in order to quickly saw or half diamond rough, especially octahedral shaped specimens.
Gemstones like jadeite and corundum are actually tougher than diamond, and while the latter may be able to scratch them, both these gem species are more resistant in terms of avoiding breakage.
Hitting or strongly knocking a diamond at certain specific angles can cause it to chip, cleave or break. Take note also that many faceted or fully-cut diamonds have a very thin girdle area. This makes them susceptible to potential damage upon impact.
Another interesting fact to consider, when talking about diamond's durability- is its carbon composition. Sure, compared to its "cousin" graphite, diamond appears to be leaps and bounds ahead in terms of resistance to damage, but consider that carbon can burn up due to the presence of extreme heat.
Diamonds caught in very high temperature fires can become cloudy or damaged (be burned). There have been instances of some stones becoming partially vaporized by great amounts of heat. For this to occur though, the diamonds also have to be in contact with air, or more specifically oxygen. Most jewellers however should be quite familiar with the limits, and can remove a diamond from the path of a specifically over-heating flame tool.
Rubies and sapphires on the other hand, can actually withstand more heat than diamond (at our normal pressure levels). They often undergo heat-exposure treatments to improve their vibrant colors.
Diamonds that form in the earth's depths do undergo exposure to crazy levels of heat, however the reason they form or exist under these temperatures is because a very high environmental pressure is present. Without this pressure, the diamonds would not survive or even form properly.
Despite diamond's innate resistance to scratching and abrasion, we would still discourage certain tests, like purposely attempting to scratch a stone against abrasive material or against other stones. Remember that even if diamond is hard, thin parts like facet junctions are relatively still in danger of chipping against directional impact. Always take care of your stones. Just because they're the hardest natural mineral on the planet, doesn't meant that they can't be damaged.
