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What is the Mohs scale of hardness?

The Mohs Scale of Hardness is a relative scale of hardness established by the Austrian mineralogist Friedrich Mohs.

He based the scale on ten minerals that are all readily available. As the hardest known naturally occurring substance, diamond is at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material.

Diamond10
Moissanite9.25
Ruby9
Sapphire9
Cubic Zirconia8.25
Spinel8
Topaz8
Aquamarine7.5
Emerald7.5
Garnet / Rhodolite7.5
Tourmaline7 - 7.5
Amethyst7
Citrine7
Lemon/Rose/Smokey Quartz7
Tiger's Eye7
Garnet6.5-7.5
Jade6.5-7
Peridot6.5
Labradorite6-6.5
Moonstone6-6.5
Agate6-7
Carnelian6-7
Onyx6-7
Opal6
Fire Opal5.5-6
Turquoise5.5-6
Hematite5-6
Lapis5-6
Fluorite4
Malachite4
Rhodochrosite4
Cultured Pearl / Mother of Pearl2.5-3.5
Amber2.5