I frequently mention the hardness of particular gemstones when I write about them. An important characteristic of a gemstone is that it is durable and hardness is one factor that needs to be considered when we talk about durability. A hard stone will not scratch or abrade easily, with the result that the facets and surface of a stone will remain polished. Hardness of a stone does not refer to how tough a stone is, as that is more about whether it will fracture easily. Toughness is another factor comprising the durability of stone.
The fact that some gemstones were harder than others was known for centuries. In his extensive ‘Natural History’, Pliny the Elder talked about the hardness of a stone as being a way to differentiate between natural and artificial gemstones. He also commented that ‘there is such a vast diversity in their degrees of hardness, that some stones do not admit of being engraved with iron, and others can only be cut with a graver blunted at the edge’.
In 1812, German geologist and mineralogist Friedrich Mohs developed a scale of gemstone hardness, based on the ability of a harder material to scratch a softer material. His scale is one we use today and is set out below. Number one, talc, is the softest material and diamond, at number 10, is the hardest.
|Mohs Hardness Scale|
|Orthoclase/feldspar (eg moonstone)||6|
|Quartz (eg amethyst)||7|
|Corundum (eg ruby)||9|
It is important to note that this scale does not mean that topaz, for instance, is twice as hard as quartz. It just means that topaz can scratch quartz, and all the other stones lower in the scale.
Over time, the scale has been extended to cover all gemstones, and half degrees have been introduced into it. It is interesting to note that pearl and coral are rated at 3, glass is rated as 6, opal also at 6, and synthetic moissanite at 9 ½.
The everyday airborne dust particles that exist around us contain a lot of sand or quartz grains so this means that jewellery containing gemstones of less than a hardness of 7 can wear more easily through ordinary use. This is why some gemstones are not suitable for rings that are worn every day, for example, but are fine for special occasion wear.