The Mohs Hardness Scale is the most widely used ranking of minerals’ hardness.
Since it’s important to be aware of how hardness affects your metalsmithing designs, it’s valuable to become familiar with Mohs.
In this guide to all things Mohs scale, we explore what it is, how you discover the hardness of stones you wish to use in your designs, and how to design to take into account a stone’s hardness.
WHAT IS THE MOHS HARDNESS SCALE?
Let’s be honest: what attracts us to a gemstone is the color and flash and beauty of the stones, not their physical properties. Lucidity Gemstones gets lots of great feedback on our stones, but no one says, “Wow! These stones had great hardness and I love that they were formed from silica and iron!”
But those physical properties are important considerations for you as jewelry makers. And the Mohs Scale is a key.
The Mohs Scale gives minerals a ranking from 1 – 10 on how hard a stone is. Quite simply, the hardness tells you how easy it is to scratch a stone. It is based on how easy it is to scratch a mineral with another mineral. For example, a topaz, with a Mohs of 8, can scratch a turquoise, with a Mohs of 5 – 6.
You’ll commonly see the Mohs designation shown with a capital ‘H’, for hardness, and a number – for example ‘agate H7’. Because stones come from different sources, hardness may be shown as a range rather than a single number. Turquoise is shown as H5 – 6 because of its many sources and variations.
While the scale is numbered 1 – 10, the increases in hardness from one number to the next are not necessarily equal. For example, diamonds are many times harder than sapphire or rubies (the corundum family).
WHO INVENTED THE MOHS HARDNESS SCALE?
The scale is named for its creator, the German/Austrian mineralogist Friedrich Mohs, who devised the scale in 1812 and published it in 1822. However, similar methods of comparing hardness have been used as far back as 300 BC, as mentioned by Theophrastus in his treatise On Stones, where he outlined physical properties of minerals.
Mohs created the scale when he acquired a part-time job working for a wealthy Austrian banker, who had a large mineral collection. The banker asked Mohs to categorize the collection.
Challenged by the lack at the time of any classification system for minerals, Mohs proceeded to create his own.
He looked for what physical properties could be used for categorization – and one of those was hardness. Mohs identified hardness based on the mineral’s resistance to scratching or how the surface of the stone will respond to contact with a sharp point.
HARDNESS VS. DURABILITY OF GEMSTONES
The durability of gemstones is central to your jewelry designs. The hardness of gems you use as a metalsmith is important, but it’s not the only factor in durability. According to the GIA (Gemological Institute of America), “Gemologists define durability as a gemstone’s ability to withstand wear, heat, light, household chemicals, and low or high humidity. Since different gemstones have different properties, they have varying tolerances to these stressors. In evaluating a gemstone’s durability, gem experts consider three factors: 1) hardness, 2) toughness, and 3) stability. No single gemstone scores a “perfect 10”.”
In addition to hardness, the resistance to scratching, the other two factors are:
Toughness – the gemstone’s resistance to breaking, cracking or chipping, a function of how hard it is to separate two surfaces of a crystal
Stability – the gemstone’s resistance to chemicals, heat, humidity and light
To see how these factors combine to indicate a stone’s durability, consider the difference between hardness and toughness of some common gemstones. While an emerald is a very hard stone, difficult to scratch, it is more easily fractured than many others.
A pearl’s overall durability is not high, especially when considering the third factor, stability, since it poorly resists chemicals such as those in perfumes and makeup.
None of these factors argue for not using certain gemstones in your designs, but rather help you determine the best uses for them – and also how you advise your customers to wear, store and clean them.
HOW TO USE THE MOHS HARDNESS SCALE
While overall durability is important in your choice of stones for your designs, hardness is probably the most pertinent. It’s not every day that someone bashes a stone, but when you consider how often you dig into a purse or pocket and encounter coins or keys, you have many occasions to scratch a gemstone in a ring. Many expensive watches have crystals made of manufactured sapphire for this very reason.
Closer to your interests as metalsmiths and artisan jewelers, keep in mind that gems softer than quartz are more likely to lose their polish and become dull even from cleaning. These stones are better reserved for designs that are less likely to get scratched or bumped, such as earrings, pendants or in statement rings that might be used sometimes but not every day.
Generally, stones of a Mohs value of 7 and above are considered hard enough for regular use in most types of jewelry without undue scratching.
If you want to know a stone’s Mohs value as you design your settings, one of the best places to find it is the International Gem Society’s list. The downside of their chart is that it is listed not by stone name, but in order of hardness. For your convenience, we’ve given you the hardness for the stones found at Lucidity Gemstones in alphabetical order.
PRACTICAL CONSIDERATION OF GEM HARDNESS IN DESIGN
Now we come to what interests you most – how to combine metalsmithing designs with the practicalities of the stones you are attracted to using. The tips below are obviously not everything you’d want to consider, but they give you a few thoughts to run through as a sort of checklist for designing around your stones’ hardness and other elements of durability.
Stick to the harder stones for jewelry that will be subject to more bumping, touching, and exposure to chemicals and elements. Think of everyday rings as being better designed of high-Mohs value stones. Jewelry that will have everyday use is best kept to stones with a Mohs value of 7 or above.
Even with higher-value Mohs stones, you can create settings that minimize exposure. An inset or bezel-set stone may give less opportunity for scratching and chipping than a higher prong-set stone. Other protective stone settings include a tension or tension-style setting, or even a modified form of a channel setting.
More delicate stones demand even more protection – lots of metal! Pearls can be set in a caged setting or bowl setting. Opals may do well with a raised prong setting, in which a bezel is augmented by prongs above the bezel, securing and protecting the opal. Amber, the softest of common jewelry stones, needs a substantial bezel if it’s to be used in a ring.
Consider what might cause unintended scratching, like bangle bracelets with stones. While silver and gold are at the low end of the Mohs scale and won’t scratch most stones, platinum or titanium might be able to scratch a more fragile stone like amber or fluorite.
Avoid putting widely different Mohs-hardness stones together in a way that they frequently bump each other. For example, you might not want to create bangle earrings that dangle both moonstone and rubies, for fear the rubies will constantly touch and scratch the moonstone.
Be aware that not all stones in a family are created equal. For example, a blue topaz, which is generally irradiated, might not exhibit the same characteristics of hardness and toughness as its natural cousins.
Finally, if you want to really consider all the aspects of gemstone durability in your metalsmithing designs, this article from the Gemological Institute of America offers two pages of helpful considerations in chart form, for most of the common gemstones.
We hope this article has been instrumental in helping you make decisions about how best to use stones in your jewelry. Remember that a lower number on the Mohs Scale doesn't mean the stone is off limits for your jewelry. You may just choose to use the stone in earrings or necklaces that are less likely to get scratched than a ring designed for daily wear.
Comments