Folks sometimes ask me to make rings out of cobalt calcite druzy or hemimorphite and I have to turn them down because the minerals are too soft. I will suggest a harder alternative like rhodocrosite or larimar or even garnet or aquamarine.
Why does it matter how hard a mineral is?
If you are going to wear it as a pendant, safely nestled against your chest, then it likely doesn’t matter – especially if the setting is protective around the mineral. However, if you are going to wear it on your finger or wrist and it’s going to get bashed around in your active lifestyle, then the harder and hardier the better. One of the reasons that engagement rings often use diamonds – the hardest mineral – is that they are expected to be worn daily for life – that is a lot of wear and tear! When storing your jewelry, you would not want a sapphire rubbing against an opal or pearls because it could easily scratch them.
How do we know how hard they are?
Like life’s challenges, how hard they are is all relative. The main measurement scale for mineral hardness was developed in the early 1800’s by Frederick Mohs. He took ten (then) relatively common minerals and ranked them in order of which ones would scratch others (or be scratched by others.)
9. Corundum (ruby, sapphire)
More currently common items and their relative hardness measures include:
gold (2.5 to 3),
platinum (4 to 4.5),
a tooth (5),
lapis lazuli (5-6),
garnet (7 to 7.5),
and emerald (7.5-8).
Note that the scale is neither linear nor logarithmic. For example, corundum is twice as hard as topaz, but diamond is almost four times as hard as corundum.
Another way I know how hard the minerals are is because I actually cut and shape many of them myself. Using a diamond saw, the softer minerals cut like warm butter but the harder ones take effort and lots and lots of patience. Carving soapstone is child’s play compared to carving agates.
What makes minerals hard or soft?
The short answer is what they are made of. The longer answer is that the molecular bonds between the atoms and the crystal structures these bonds form determine their hardness. While each mineral always has the same chemical composition (e.g. quartz is always SiO2), it can come in many different crystal structures. Carbon offers the broadest range of hardness as it comes in graphite (pencil lead that scrapes easily onto paper) and diamond. The only difference is the form of the lattices or structures that the carbon atoms form in each.
While not technically related to their hardness, friability or fragility can also be an issue to keep in mind. Druzy specimens with lots of tiny crystals could crumble if the crystals are not well attached to each other. Some crystals also have very strong cleavage plains and are susceptible to breaking along those angles. Lastly, some minerals are reactive to acids or other agents and need to be sealed or otherwise protected.