Mohs' Hardness Scale

Mohs took ten well known, easily available minerals, and arranged them in order of their "scratch hardness". If a specimen to be tested can be scratched by a known mineral from the list, it is softer than that mineral. If it in turn will scratch another known mineral, it is harder than that mineral. This gives a very quick and easy field test for hardness. As such is it very useful for mineralogists. It is too destructive to be commonly used in gemology, but is available and can be valuable on rough gemstones. A crude form gets used on shop windows by people testing out their diamond rings, in the mistaken belief that only diamond will scratch glass.

There are many different aspects of materials which could be considered as a measure of hardness. Hardness can mean resistance to scratching, indentation, bending, breaking, abrasion, cleavage, or fracture. It is easy to confuse durability or toughness with hardness. A very simple example is to consider a glass ball and a rubber ball. Glass is harder than rubber, but rubber is more durable. Try bouncing both on a hard floor, the glass ball will shatter, whereas the rubber ball will bounce. The aspect of hardness which is measured by Mohs' test is the scratchability of a mineral.

Other scales of hardness include Brinell's Vicker's, Knoop, Meyer, Rockwell, and a rebound (dynamic or absolute hardness) test measured with a sclerometer. The hardness of each mineral relative to the others varies according to which test is performed, and also hardness can vary according to the grain direction, or crystallographic orientation, of the specimen. We have given typical Brinell hardness figures for each of the minerals in the table. Diamond's hardness cannot be measured on the Brinell scale, because a diamond indenter is used for the test itself, but it is many times harder than corundum. Sapphire and ruby are the well known varieties of corundum. Amethyst, citrine, rock crystal, and cairngorm are all varieties of quartz.

Generally, high scratch resistance is desirable for gemstones, and a Mohs' hardness of 7 or higher is important. The principal reason is that a common cause of abrasion is sand, which is silica grit (quartz), and is commonly present in dust. Stones which are softer than quartz are not suitable for everyday use as faceted jewelry gemstones, particularly in rings, although many are beautiful and attractive.

Some gems, such as pearls, coral, turquoise, lapis lazuli, amber, and opal are quite soft, but are usually polished into cabochons or beads, rather than faceted, and therefore do not show scratches so easily. All these gemstones have been successfully used in jewelry for many centuries.