You’ve just been gifted a glittering ring, or perhaps you chose a lovely pendant for yourself. It’s made of gold and diamonds, and you know that gold is pretty soft, but diamond is practically indestructible! But only a few weeks later, tragedy strikes – your brand-new jewelry falls onto a hard floor, and the diamond cracks right in half. What happened?
While diamond is no longer the hardest natural mineral found on Earth (it lost its title to both lonsdaleite and wurtzite boron nitride in 2009), it’s still considered the standard for ultra-hard materials. For most purposes, this means it’s extremely durable, but clearly there’s something else at play here! Don’t worry, you weren’t fooled by a fake: diamonds aren’t actually all that tough.
To understand why your diamond cracked so easily, you must differentiate between hardness and toughness. Hardness is a material’s resistance to scratching, while toughness defines how readily a material will chip or crack (it may be easier to think of it as “brittleness”). Diamonds are very hard, but relatively brittle because of their cleavage.
So what is cleavage, anyway? No, not that. Cleavage is a property of most minerals caused by planes of weakness in their atomic structure. If you were a rockhound as a kid, you may recall finding glittering sheets of mica and peeling them apart. This is an extreme example of cleavage; the atoms in mica are so weakly bonded along those planes that the mineral splits into sheets, which theoretically could be only an atom thick!
Thankfully, girls’ best friend doesn’t cleave nearly as easily as mica. It takes a pretty hard hit to break a diamond, and it’ll only break if that hit is at just the right angle – the cleavage planes follow the octahedral shape traditionally associated with diamond. Normally, diamonds are cut so that an accidental strike to the girdle, or edge, of the stone won’t damage it. But sometimes a diamond must be cut in a way that exposes a cleavage plane, and a sharp blow, like a long fall to the floor, could be exactly what does your diamond in.
Unfortunately, once your diamond has broken, there’s no fixing it – hopefully you have your jewelry appraised by a GIA Graduate Gemologist Appraiser and insured with a personal article policy! – but the calamity is easy to avoid. Keep your jewelry safely stored in a padded box or pouch, well away from the edges of tall furniture (especially if you have cats), and don’t wear it if you might hit it against something hard. So while it’s tempting to show off your brand new, sparkly engagement ring to your girlfriends at the gym, consider leaving it at home this time and gushing about it over coffee instead!