In 1822, Friedrich Moh, a German mineralogist established a practical method of comparing hardness or scratch resistance of minerals. It has become universally known as Moh's scale. The Mohs’ hardness scale places ten common or well-known minerals on a scale from one to ten. One is the softest mineral and ten is the hardest. These are the minerals used in the Mohs’ hardness scale:

Mohs' Hardness Scale

1	2	3	4	5
Talc	Gypsum	Calcite	Fluorite	Apatite

 

6	7	8	9	10
Feldspar	Quartz	Topaz	Corundum	Diamond

 

Measures of Hardness:

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 Moh's test is the scratchability of a mineral.

Importance for Gemstones:

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 (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.