The Mohs scale was developed in 1812 by a German mineralogist named Freidrich Mohs. It is one of the ways in which the hardness of minerals is measured.
The scale includes ten readily-available reference minerals, ranked from 1 to 10 by their hardness. The lowest-ranked is talc and the highest is diamond, which was the hardest mineral known in 1812.
It is not a linear scale – diamond is four times harder than corundum – but a ranking scale, comparing minerals to each other.
- How the Mohs Scale Works
- Why is the Mohs Scale Important?
- Estimates Using Common Objects
- Limitations of the Mohs Scale
- The Importance of Hardness in Gemstones
- Where Common Gemstones Rate on the Mohs Scale
How the Mohs Scale Works
You find the Mohs ranking of a mineral sample by taking 2 minerals – one from the scale and the mystery mineral – and trying to scratch one with the other. Hold the mystery mineral firmly on a flat surface, with a smooth, unscratched side facing up. Take the testing mineral and firmly scratch a pointed bit of it across the surface of the mineral you are testing. Brush away any dust and look for scratches.
If A scratches B, A is harder. In the scenario that A does not scratch B, then B is harder. If neither one is scratched, they are about the same. If A scratches B but doesn’t scratch C, then the hardness of A is between B and C.
Repeat the test to confirm your results. Continue through the sample kit until you find the point where one sample doesn’t scratch and the next one does. Your mystery sample’s hardness ranking is between those two. Harder minerals – the ones in the upper half of the scale – can require forceful pressure to test. Be careful to scratch away from yourself and your fingers.
The ten reference materials are:
- Orthoclase feldspar
Why is the Mohs Scale Important?
Field geologists often carry a scratch kit containing samples of the ten reference minerals, or a set of picks called “hardness picks” that are scaled to the Mohs scale. The kits also include a list of rocks and minerals and where they fall on the scale. They use the scale to help identify minerals in the field.
The Mohs scale can also help you know how you need to care for the minerals used in your jewelry. Minerals at different points in the scale require different care. You will need to be careful what you do with softer gemstones – opals are easy to break even just by overheating them (doing the dishes, wearing in a hot tub) or hitting them against something.
Estimates Using Common Objects
If you know the Mohs scale of some common objects you are likely to have available, you can make a quick estimate even without a kit.
Your fingernails are about 2.5. A copper penny is a 3 – their softness is why the dropped pennies you find in parking lots are all scratched up from being ground against the concrete. A pocket knife is 5 – 6.5, and a piece of glass is 5.5. A steel file is a 6.5.
As noted in the scale, quartz is a 7, and there is almost always a piece of quartz lying in the gravel nearby.
Limitations of the Mohs Scale
While the Mohs scale can tell how hard a stone is, that does not mean that stone is durable. Hardness and durability (toughness) are different; a hammer that comes in at 5.5 on the scale can shatter a diamond if it hits at the right angle. Durability has to do with the crystalline structure of the gemstone. Several kinds of mineral have different crystalline structures – quartz is a good example. The various structures can be the same hardness, but different durability.
Gemstones are also often treated with oil or resins to enhance color and decrease flaws. These treatments can be sensitive areas of the stone. To really know the best way to care for your gemstones, you need to know both scores – how hard and how durable – your kind of stone is, as well as any treatments it has received.
The Importance of Hardness in Gemstones
Some gemstones are more appropriate for jewelry worn in more protected areas – necklaces, pendants, earrings – instead of in areas likely to be exposed to wear and tear like rings or bracelets. Gemstones worn in jewelry that is likely to be bumped into surfaces like door handles, countertops, or other jewelry should be at least a 7, which is the point at which it is unlikely to be scratched by common mineral dust.
The hardest mineral dust that is likely to be airborne is quartz – and quartz particles in dust are very common. Say you have a turquoise bracelet, and it gets dirty. You get a soft cloth to wipe it, but some of the dust is harder than the stone. Over time, it becomes dull and scratched up. It is essential to rinse gemstones, especially the softer ones, with warm water to get the dust off before wiping them off. Knowing the hardness of your stones will guide you in caring for them.
Where Common Gemstones Rate on the Mohs Scale
Let’s look at the hardnesses of gemstones commonly used in jewelry.
1 – 6 on the Mohs Scale
- 2 – 2.5. Amber is fossilized tree sap, and the softest regularly used gemstone. It is delicate and can be harmed by many common substances like gasoline, alcohol, and cleaning chemicals. It should never be cleaned with an ultrasonic cleaner.
- 2.5-4.5 – Pearls. Due to the way they are created – layer by later inside a mollusk – they are easily broken and may dissolve in even mild acids. Cleopatra famously dissolved a pearl in vinegar and drank it to win a bet. Chlorine, perfume, and makeup can stain or damage pearls. You should clean them with a mild soap and distilled water (tap water contains chlorine).
- 5 -Multiple gemstones have a hardness between 5 and 6.
- Lapiz lazuli is a 5. Turquoise is 5-6. Turquoise is absorbent. If exposed to liquids like oils (say, worn while cooking), perfumes, or detergents, the color deteriorates you should clean these using only very mild soap and pure water.
- Jade at 5-6.5 is a soft, but very tough gem. It is easy to scratch but is also easy for an artist to carve into gorgeous designs.
- 5.5-6.5 is the opal. Opals need moisture – they are up to 30% water – and if they dry out, they will crack and fade. They can break or chip easily.
6 – 10 on the Mohs Scale
- 6 – Moonstone and Sunstone, both varieties of feldspar, are 6 on the Mohs scale. Avoid harsh chemicals, steam cleaning, or ultrasonic cleaning. Mild soap and non-chlorinated water are all that you should use to clean them.
- 7 – Many kinds of gemstones come in between 7 and 8 on the Mohs scale.
- Quartz has several possible crystalline structures and comes in many colors. These include amethyst, tiger’s eye, rose quartz, chalcedony, agate, carnelian, and citrine. Ultrasonic cleaners and steamers are safe with many kinds of quartz, though you should use mild detergent and warm water instead if you can.
- Garnets range from 7 to 7.5. They are sturdy, but should still be cleaned with mild soap and water and kept from scratches from harder materials.
- Beryl ranges from 7.5 to 8 and includes aquamarines and the hard but fragile emerald. They are sensitive to household chemicals and hard knocks, and you should treat them gently.
- 8 – Topaz is one of the exemplars on the scale, harder than quarts and softer than corundum. It is not easily scratched, so you can wipe it off without worrying as much.
- 9 – Corundum includes rubies and sapphires. Washing and wiping corundum isn’t a problem as they are tough and durable. Avoid harsh chemicals even with very hard gemstones.
- And of course, diamonds are the perfect 10 of the scale. (There are a few minerals that are actually harder than the diamond, they are rare, often man-made, and used for industrial purposes.)
The Mohs scale is an excellent way to quickly narrow down the kind of mineral you are holding and give a rough idea of some of the care it should have. It’s not the whole picture of that stone’s strengths and weaknesses, but it’s a useful start.