Gemstone Hardness: The ability of a gemstone material to resist scratching and abrasion

 

Gemstone hardness indicated in Mohs’ scale

Gemstone hardness is one of the factors on which the durability of a gemstone depends. In gemmology, hardness is defined as the ability of a gemstone to resist scratching and abrasion. In relation to gem set jewellery, gemstone hardness is important as it indicates how well a gemstone will keep its polish during wear. The higher the hardness of a gemstone, the higher polish it will take and the longer it will retain that polish.

(NOTE: The hardness of a gemstone is a surface property and should not be confused with gemstone toughness: gemstone toughness is a body property which indicates the ability to resist fracture or cleavage.)

To indicate the hardness of a gemstone we use the Mohs’ scale of relative hardness. For Mohs’ scale, ten common minerals have been brought together in a list. Each material has its own consistent composition and hardness and they have been put in to order from 1 (extremely soft) to 10 (extremely hard). A mineral with a lower number will be scratched by one with a higher number. And of course vice versa: a mineral with a higher number will be able to scratch one with a lower number on the Mohs’ scale.

The hardness on Mohs’ scale indicates an order of relative hardness. For example: topaz, corundum and diamond are adjacent on Mohs’ scale, respectively with hardness 8, 9 and 10. But the difference in hardness between corundum (9) and diamond (10) is far greater than that between topaz (8) and corundum (9).

Mohs’ scale

The Mohs’ scale list comprises the following minerals:

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar (Orthoclase)
7. Quartz
8. Topaz
9. Corundum
10. Diamond

Gemstones set in rings

Many things that surround us in daily life have a hardness of 7 or less. Therefore it’s generally recommended to choose a gemstone with hardness 7 or higher for the use in rings. This does not mean that gemstones with lower hardness can’t be set in rings at all, but it means that the wearer should be aware of this. Stones with a hardness below 7 might be more suitable for dress rings and less suitable for rings that will be worn on a daily basis.

NOTE: Gemstone rings should always be worn with care, no matter how hard the gemstone that was chosen. Even diamond can get damaged if it’s not taken care of properly!

Mohs’ scale and relative gemstone hardness of other (gem) materials

Of course, as a goldmsith or jewellery designer you will come across many more materials than just those ten that have been included in the Mohs’ scale. An overview of different gemstone materials (but also other ordinary materials as a reference) relative to the minerals in Mohs’ scale:

1. Talc
   • 1½. Steatite (soapstone)
2. Gypsum
   • 2½. Amber, Ivory, Fingernail
3. Calcite, ‘Copper’ coin
   • 3½. Pearl, Coral, Jet
4. Fluorite
5. Apatite
   • 5½. Window glass, Bottle glass
6. Feldspar (orthoclase), Moonstone, Opal, Steel blade
   • 6½. Demantoid garnet, Nephrite, Peridot, Tanzanite
7. Quartz, Tourmaline, Jadeite, Hard steel file
   • 7½. Pyrope, Almandine and Hessonite garnet, zircon, beryl
8. Topaz, Spinel
   • 8½. Chrysoberyl
9. Corundum
   • 9½. Synthetic moissanite
10. Diamond

Gemstone Hardness: Alphabetical list of gemstone materials and ornamental materials often used in jewellery

• A
  • Amber – 2½
  • Andalusite – 7½
  • Apatite – 5
• B
  • Beryl varieties – 7½
  • Bowenite (see “Serpentine)
• C
  • Calcite varieties – 3
  • Chrysoberyl – 8½
  • Corundum varieties – 9
  • CZ (cubic zirconia) – 8 to 8½
• D
  • Diamond – 10
  • Diopside – 5½
• F
  • Feldspar varieties – 6
  • Fluorite – 4
• G
  • Garnet (almandine) – 7½
  • Garnet (demantoid) – 6½
  • Garnet (grossular) – 7¼
  • Garnet (hydrogrossular) – 7¼
  • Garnet (pyrope) – 7¼
  • Garnet (spessartine) – 7¼
  • Gypsum varieties – 2
• H
  • Hematite – 5½ to 6½
• I
  • Iolite – 7 to 7½
  • Ivory, dentine – 2 to 3
  • Ivory, vegetable – 2½
• J
  • Jadeite – 7
  • Jet – 2½ to 4
• L
  • Lapis lazuli – 5½
• M
  • Malachite – 4
• N
  • Natural glass – 5 to 5½
  • Nephrite – 6½
• O
  • Opal – 6
• P
  • Paste (artificial glass) – approx 6
  • Peridot – 6½
  • Pyrite – 6½
• Q
  • Quartz (crystalline) – 7
  • Quartz (polycrystalline) – 6 to 7
• R
  • Rhodochrosite – 4
  • Rhodonite – 6
• S
  • Scapolite – 6
  • Serpentine, bowenite – 5
  • Sinhalite – 6½
  • Sodalite – 5½ to 6
  • Sphene – 5 to 5½
  • Spinel – 8
  • Spodumene – 7
  • Steatite – 1
  • Synthetic Moissanite – 9¼
• T
  • Tanzanite – 6½
  • Topaz – 8
  • Tortoiseshell – 2½
  • Tourmaline – 7 to 7½
  • Turquoise – 5½ to 6
• Y
  • YAG (Yttrium Aluminium Garnet) – 8
• Z
  • Zircon – 6½ to 7½

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