Friedrich Mohs, a German mineralogist, devised a crude but practical method of comparing the hardness of scratch resistence of minerals. It should more accurately be called a table, because it is not to scale, that is the numbers allocated to different minerals are not proportional to their actual scratch resistance, so that the scale is really an ordered list.The aspect of hardness which is measured by Mohs' test is the scratchability of a mineral.

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 facetted jewellery 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 facetted, and therefore do not show scratches so easily.

Mohs scale is based upon the fact that a harder material will scratch a softer one. On the Mohs scale 10 is the hardest and 1 is the softest.

The hardness of some familiar items:

Fingernail: 2.5
Penny: 3
Glass: 5.5

So, to figure out the hardness of a mineral, scratch it with an item of known hardness. For example, if a fingernail cannot scratch a mineral, we know it has a hardness of more than 2.5. If the mineral can't scratch glass, we know it has a hardness of less than 5.5.