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Gemstone and Hardness

HARDNESS

For a gem, hardness equals permanence. A gem’s hardness has important consequences for the way it’s used. Scratches show up more on faceted stones than on cabochon-cut gems like opal, moonstone, and turquoise. A gem crystal’s hardness is directly related to the strength of the chemical bonds between its atoms. In general, the stronger the bonds, the harder the crystal. Gemerally speaking, a hard stone takes a better polish than a soft one. A hard stone like sapphire, for example, has sharper facet edges and a better quality surface reflection than a softer one like peridot.

Hardness is usually expressed in terms of the Mohs scale. On the Mohs scale, diamond is the hardest, at 10, and talc is the softest, at 1. The steps between the minerals on the Mohs scale don’t represent uniform differences in hardness. They merely represent an order of relative hardness. Each mineral can scratch the ones below it on the scale. Frequently stones will be found that in hardness are between some two of Mohs's hardness scale. In that case we add one half to the number of the softer mineral; thus, peridot, benitoite, and jade (nephrite) are all softer than quartz (7) but harder than feldspar (6); hence we say they are 6.5 in hardness. Beryl (aquamarine and emerald), garnet (almandite, pyrore-almandite), and zircon are rated 7.5 in hardness, being softer than topaz (8) but harder than quartz (7). 

MOHS'S HARDNESS SCALE

Mohs's Hardness Scale

Comparative gems or minerals

10

Diamond

9

Corundum (ruby, sapphire)

8

Chrysoberyl, Spinel, Topaz

7

Quartz (Amethyst, Citrine, Prasiolite)

6

Orthoclase Feldspar

5

Apatite

4

Fluorite

3

Calcite

2

Gypsum

1

Talc (Soap stone)

Quartz, with a hardness of 7, has an important place in the gem hardness scale because most of the dust in the air is composed of quartz. Eventually, a cleaning cloth becomes impregnated with dust. The simple act of wiping a soft stone like chrome diopside can damage its surface and dull its polish. Having an idea of what hardness means and how it is expressed, we must next inquire how one may make use of it in determining the best choice stones for your jewelry:

a.) Diamond is harder than any other gemstone.

b.) Ruby and sapphire make excellent ring stones.

c.) Chrysoberyl, topaz, spinel, emerald, and aquamarine have a hardness between 7 1/2 and 8 1/2 and make very good ring stones. (Topaz and emerald have other durability challenges: topaz due to cleavage, emeralds due to inclusions and treatments.)

d.) Tourmaline, tsavorite, red garnets, iolite, amethyst, and zircon have a hardness between 7 and 7 1/2, and make good ring stones. (Most dust in the atmosphere is quartz, with a hardness of 7. Gems below 7 on the Mohs scale will suffer abrasion during cleaning and wear).

e.) Kunzite, demantoid garnet, peridot, and tanzanite have a hardness between 6 1/2 and 7. They’re best suited for items such a pins, earrings, and pendants, where they won’t be so exposed to damaging situations. When mounted in rings, these stones require care when worn.

f.) Nephrite and jadeite have a hardness between 6 and 7. In spite of this, they make good ring stones. This is because their aggregate structure makes them extremely tough—especially important for men’s jewelry. Chalcedony, with a hardness of 6 1/2 to 7, is also tough for the same reason.

g.) Rhodonite, Feldspar, turquoise, non-phenomenal opal like fire opal, and lapis lazuli have a hardness between 5 and 6, and are best suited for items such as earrings and pendants. Wear is less visible on these stones because of their lack of transparency and the way they are cut.

h.) Pearl, rhodochrosite, malachite, coral, amber, and ivory rate between 2 and 4 1/2 on the Moh’s scale. They’re best suited for items such as pins, earrings, and pendants, or for use as decorative objects. These gem materials can be damaged or discolored by chemicals or application of heat that would leave most other gems unharmed.