Gemstone Hardness and How Does It Affect Your Jewelry?

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Often I was asked by customers how durable a gemstone is, or will it get damaged easily. One main factor of determining whether the gemstone can stay shiny and polished for a long time, or the general longevity of it, is to look at its hardness.
Hardness of a gemstone refers to its ability to withstand scratches, or its “scratchability”. Gemstones with a higher hardness can stay shiny and new for a longer time (more on that later).

What is gemstone hardness?

Mohs scale of hardness is widely used to measure the hardness of gemstones. Created by German mineralogist Friedrich Mohs in 1822, each gem is assigned a hardness number. A gem is able to scratch another one with a lower hardness number. So a gem with hardness of 7 can scratch a gem with a hardness of 6.

These 10 gems are selected to represent the scale from 1 to 10 because they can easily be obtained in a highly pure form.

Mohs scale of hardness and the corresponding minerals

For example, fingernails have a hardness of about 2.5, and can scratch gemstones like talc (1) and gypsum (2). A knife, with a hardness of about 5.5, will scratch gems like fluorite (4) and apatite (5).
However, using hardness test is not common. Who would want a scratch on a beautifully faceted moonstone? So testing hardness by scratching a stone is mainly for unpolished minerals.
Hardness Picks

Why are hardness numbers important?

One of the most important hardness number is 7, which is the hardness of quartz. The air around us is filled with dusts and other small airborne particles. Those microscopic particles are mostly made of quartz and silica, which have a hardness of 7. They can scratch any gemstones with a hardness lower than 7.
Although the scratches are small, they will build up over time and take away the shine and polish on the gemstone’s surface. Daily actions like putting your hand in the pocket will rub a gemstone against fabrics and cause some small scratches. Gemstones used in rings and bracelets will be more exposed to scratches than pendants and earrings. That is why hardness contributes to a part of the durability of the gems.
Higher hardness gemstones means better ability to withstand microscopic scratches and stay shiny for longer. It’s one of the reasons why gems with higher hardness like ruby, sapphire and topaz are popular choices for jewelry.

Does high hardness means the gem is not breakable?

Hardness doesn’t tell whether the stone is breakable. Whether the gem is breakable is determined mostly by the crystal structure.
Being the hardest gem, diamond can still break when knocked at certain angles. Story goes in the early days of prospecting for diamonds, many struck a sledgehammer to test whether the diamond is real. Of course many of the diamonds “tested” didn’t survive the blow, and a lot of the fine stones were destroyed by this method.
On the other hand, nephrite jade only has a hardness of 6.5. But it’s one of the toughest gemstones because of its internal structure. 

Then does it mean softer gemstones are not suitable for jewelry?

Not necessarily. Softer stones, like lapis lazuli and chrysocolla, will require more care when wearing. Avoid physical activities when wearing them. Protective settings will also help prolong the lifetime of these gemstones.
When cleaning gemstones, hardness also plays a part. Use a soft cloth to clean your gemstones because a rough one will scratch the softer gemstones. A soft cloth is recommended as it keeps the scratches to a minimum.
Opal ring with protective setting

Here are a list of popular gemstones with their hardness numbers:

Agate 7
Alexandrite 8.5
Amazonite 6-6.5
Amber 2-2.5
Amethyst 7
Ametrine 7
Ammolite 3.5-4
Angelite 3.5
Apatite 5
Aquamarine 7.5-8
Aragonite 3.5-4
Aventurine 7
Azurite 3.5-4
Beryl 7.5-8
Bloodstone 6.5-7
Bronzite 5-6
Calcite 3
Carnelian 7
Celestine 3-3.5
Chalcedony 7
Charoite 4.5-5
Chrome Diopside 6
Chrysoberyl 8.5
Chrysocolla 2-4
Chrysoprase 7
Citrine 7
Copper 2.5-3
Coral 3.5
Cordierite 7-7.5
Corundum 9
Cuprite 3.5-4
Diamond 10
Diopside 6
Dumortierite 7-8
Elbaite 7-7.5
Emerald 7.5-8
Epidote 6-7
Feldspar 6
Fluorite 4
Fuchsite 2.5
Garnet 6.5-7.5
Gold 2.5-3
Gypsum 2
Halite 2.5
Hawk’s Eye 6.5-7
Heliodor 7.5-8
Heliotrope 6.5-7
Hematite 5-6
Hemimorphite 4.5-5
Hiddenite 6.5-7
Howlite 3.5
Indicolite 7-7.5
Iolite 7-7.5
Iron 4
Jade (Jadeite) 6-7
Jade (Nephrite) 6.5
Jasper 7
Jet 2.5
Kunzite 6.5-7
Kyanite* 4.5;6
Labradorite 6-6.5
Lapis Lazuli 5-5.5
Larvikite 6-6.5
Larimar 4.5-5
Lepidolite 2.5-3.5
Malachite 3.5-4
Marcasite 6-6.5
Microcline 6-6.5
Moldavite 5.5
Mookaite 6.5-7
Moonstone 6-6.5
Morganite 7.5-8
Muscovite 2.5
Nephrite 6.5
Obsidian 5-5.5
Olivine 6.5-7
Onyx 7
Opal 5-6
Pectolite 4.5-5
Peridot 6.5-7
Petrified Wood 6.5-7
Phosphosiderite 3.5-4
Pietersite 6.5-7
Prehnite 6-6.5
Pyrite 6-6.5
Quartz 7
Rhodochrosite 3.5-4
Rhodonite 6
Rock Crystal 7
Ruby 9
Ruby Zoisite** 6.5-7;9
Rutile 6-6.5
Sapphire 9
Sardonyx 7
Scapolite 5-6
Schorl 7-7.5
Selenite 2
Seraphinite 2-4
Serpentine 3.5-5.5
Silver 2.5-3
Smithsonite 4-4.5
Sodalite 5.5-6
Sonora Sunrise/Sonora Sunset 2-4
Sphene 5-5.5
Spinel 7.5-8
Spodumene 6.5-7
Sugilite 5.5-6.5
Sunstone 6-6.5
Talc 1
Tanzanite 6-7
Tiger’s Eye 7
Topaz 8
Tourmaline 7-7.5
Turquoise 5-6
Unakite 6-7
Verdite 3
Zircon 7.5
Zoisite 6-7
* Kyanite has a special property of having two distinct hardness (directional hardness). The lower hardness value runs along the crystal’s length, while the higher hardness is across the crystal’s width.
** Ruby zoisite has two different hardness because it consists of both red ruby (with a hardness of 9) and green zoisite (with a hardness of 6-7) in one mineral.