Gemstone hardness – Mohs scale of mineral hardness
In 1812 a German mineralogist by the name of Friedrich Mohs developed a scale of hardness based upon a harder minerals ability to scratch a softer one. Although quite an aged scale, Mohs scale is still used in the modern jewellery trade for describing hardness in gem material.
| Mohs hardness | Mineral | Absolute hardness |
|---|---|---|
| 1 | Talc | 1 |
| 2 | Gypsum | 3 |
| 3 | Calcite | 9 |
| 4 | Flourite | 21 |
| 5 | Apatite | 48 |
| 6 | Orthoclase Feldspar | 72 |
| 7 | Quartz | 100 |
| 8 | Topaz | 200 |
| 9 | Corundum (Rubies and Sapphires) | 400 |
| 10 | Diamond | 1600 |
Mohs scale is an ordered scale only, in that there is no relation to the hardness of a consecutive pair of minerals to say that of the previous or next. For example, sapphires are two times harder than topaz, yet diamonds are around four times harder then sapphires. The scale is based solely on the ability of one gemstone to scratch another softer stone. Ten minerals have been selected for the scale—there are however many other gemstones that are represented through one of the Mohs ten.
A gem stone by definition needs to be three things—durable, valuable and attractive. Mohs scale is related to the durability of a gem stone. If a stone is expensive, beautiful but soft it becomes impossible to wear without damaging it. There are many gemstone collectors who would be up at arms at the statements in this paragraph. At Metal Urges we are in the business of providing our clients with the highest quality handmade jewellery possible. We only sell stones of hardness 8 and higher. Our most popular are diamonds, which are the hardest naturally occurring mineral on the planet. Corundum, represented by both ruby and sapphire and some other hard exotic stones such as alexandrite, chrysoberyl and spinel.
The simple truth about hardness is this:
The dust in air by and large is made up of powdered quartz particles. These quartz particles will abrade and damage any surface with a lower hardness. So it’s fair to say that even the very air we breathe slowly but surely damages gem materials with a hardness or 7 or lower. For your jewellery to stand the test of time then use a stone of harness 8–10 only. At Metal Urges we only sell handmade jewellery containing hard stones such as diamond, sapphire, ruby and alexandrite.
Hardness vs Toughness
Mohs scale relates to ‘hardness’ and not ‘toughness’. Hardness depicts a materials ability to stand up to wear or abrasion, typically the harder a material is the more stiff or brittle it becomes. Toughness is a measure of the ability of a material to absorb energy, and is a measure of how likely a material is to snap, chip or shatter. An analogy previously mentioned in 18ct gold vs. 9ct gold compares glass to perspex. Glass wont scratch easily, but is not flexible and is prone to shattering. Perspex is a tougher material in that it can be bent as well as absorb an impact, yet it is not very hard, as it can be easily scratched. Hardness and toughness are often directly related, in that a material that rates well with one is generally poorer with the other. There are definite gemstone parallels with this line of thought. Stones such as Columbian Emerald and Topaz are hard but extremely brittle, making them best suited for earrings, pendants and other low wear pieces.
Diamonds
Many believe diamonds to be indestructible. They are one of the hardest know materials, and will endure amounts of surface wear and tear. This is great for jeweller, as the brightness and sparkle of a diamond will usually last a lifetime. People are often surprised to learn that diamonds can be broken. As mentioned previously, hardness doesn’t necessarily mean toughness. While not as brittle as many other gemstones, diamonds can still be chipped or shattered, care must be taken to not expose them to high impacts or leverage.
Cleavage
To make things even more confusing, gemstones have another property called ‘cleavage’. Cleavage is the crystal tendency to break along a particular plane. Gemstones have either none or 1 to 5 planes of cleavage. Each of these planes then also a rating which relates to the ease at which that crystal will break along its plane. The number of planes of cleavage and cleavage rating is specific to each gemstone, and is something which both gemstones cutters and stone setters must take into account. Metal Urges Fine Jewellery are qualified gemologists and understand the nature of each gemstone and its particular properties. This allows Metal Urges to work safely with your family heirlooms.
Corundum
Sapphires and rubies are members of the corundum family. While not as hard as diamonds, sapphires and rubies have no planes of cleavage. This doesn’t mean they won’t chip or shatter in extreme situations, but does make them well suited for use in your custom jewellery.
More professional and skilled Jewellers make conscious decisions regarding which gemstones are best suited to which type of jewellery based upon a stones hardness and toughness, and therefore its ability to stand up to a given amount of wear. Here at Metal Urges Fine Jewellery we manufacture custom wedding bands and engagement rings using mainly sapphires and diamonds. We make this decision based upon our knowledge and experience with gemstones, and our desire to create handmade jewellery pieces which will stand the test of time and go on to become heirloom pieces for future generations.
Often we are approached by clients requesting we create a custom ring utilising a stone which we deem to have poor qualities in the above mentioned areas. Occasionally with the right stone, we make exceptions to our rules, but this often impacts on design as special consideration has to be made to protect the stone. Some gemstones simply aren’t suited to engagement rings.