Minerals, including gemstones, are rated and ranked according to their hardness. While hardness is generally associated with durability, the ability to resist breakage is better described as toughness. When referring to gemstones, hardness more accurately means the stone’s ability to resist abrasion.

Regardless of which scale is used, the diamond is considered the hardest substance known to man. The most common measure of a gemstone’s degree of hardness is based on the Mohs Scale. Devised by German mineralogist Friedrich Mohs, the Mohs Scale grades minerals on a comparative scale from 1 (very soft) to 10 (very hard).

What the scale means is that a mineral of a given hardness rating will scratch other minerals of the same rating, as well as any minerals of a lower hardness rating. For example, rubies and sapphires, which are composed of the mineral corundum and have a Mohs rating of 9, will scratch each other, as well as topaz (rating 8) and quartz (rating 7). But they will not scratch diamonds, which are rated 10.

Hardness

Mineral

Associations and Uses

1

Talc

Talc Talcum powder.

2

Gypsum

Plaster of paris. Gypsum is formed when seawater evaporates from the Earth’s surface.

3

Calcite

Limestone and most shells contain calcite.

4

Fluorite

Fluorine in fluorite prevents tooth decay.

5

Apatite

When you are hungry you have a big “appetite”

6

Orthoclase

Orthoclase is a feldspar, and in German, “feld” means “field”.

7

Quartz

Silica for glass, electrical components, optical lenses, abrasives, gemstones, ornamental stone, building stone, etc.

8

Topaz

The November birthstone. Emerald and aquamarine are varieties of beryl with a hardness of 8.

9

Corundum

Sapphire and ruby are varieties of corundum. Twice as hard as topaz.

10

Diamond

Used in jewelry and cutting tools. Four times as hard as corundum.

 

What the scale does not make obvious is that the numeric values assigned to each interval of hardness are not equal. Some stones are disproportionately harder than others:

Gemstone

Mohs Rating

Results from Grinding Test

Diamond

10

Approximately 90 times harder than corundum

Ruby/Sapphire (corundum)

9

Approximately 5 times harder than topaz

Topaz

8

Little difference between topaz and quartz

Quarts

7

Harder than glass.