Facts & Figures

There are three important measurements of a gem’s physical and optical properties:


  • HARDNESS: Devised in the 18th century by Viennese mineralogist Friedrich Mohs, The Mohs’ Hardness Scale measures the ability of a gem to resist surface scratching. Mohs quantified hardness on a scale of 1 to 10, with 10 being the hardest and 1 the softest. As the minerals chosen set the levels of hardness, it is a comparative not a relative scale. The hardest mineral is Diamond, followed by Corundum (Ruby and Sapphire) then Topaz.
  • REFRACTIVE INDEX: Developed by Dutch mathematician Willebrord Snellius (Willebrord Snel van Royen) in the 17th century, the refractive index measures a gemstone’s brilliance. Critical in gem identification, a gem’s refractive index is assessed by using a refractometer to measure the speed and angle of light as it enters the gem.
  • RELATIVE DENSITY: Due to differences in their Chemical composition and crystal structure some gem types are heavier than others. Relative density (grammes per cubic centimeter on a 1 to 8 scale) has now largely replaced specific gravity (the ratio of the weight of a specific material to the weight of the same volume of water) as the index used to measure gemstone density.

GemstoneHardnessRefractive indexRelative density
Aquamarine 7.5 - 8 1.564 - 1.596 2.68 - 2.74
Agate 6.5 - 7 1.530 - 1.540 2.60 - 2.64
Alexandrite 8.5 1.746 - 1.763 3.70 - 3.78
Almandine 6.5 - 7.5 1.83 4.3
Amazonite 6 - 6.5 1.522 - 1.530 2.56 - 2.58
Ambligonite 6 1.578 - 1.646 3.01 - 3.11
Amber 2 - 2.5 1.539 - 1.545 1.05 - 1.09
Amethyst 7 1.544 - 1.553 2.65
Andalusite 7.5 1.627 - 1.649 3.05- 3.20
Andradite 6.5 - 7.5 1.88 - 1.94 3.70 - 4.10
Anglesite 3 - 3.5 1.880 - 1890 6.30 - 6.39
Apatite 5 1.628 - 1.649 3.16 - 3.23
Aragonite 3.5 - 4 1.670 - 1.690 2.94
Aventurine 7 1.544 - 1.553 2.64 - 2.69
Azurmalachite 3.5 - 4 1.655 - 1.909 3.25 - 4.10
Yellow Beryl 7.5 - 8 1.562 - 1.602 2.66 - 2.87
Bixbite 7.5 - 8 1.562 - 1.602 2.66 - 2.87
Bronzite 5.5 1.650 - 1.680 3.20 - 3.30
Chalcedony 6.5 - 7 1.530 - 1.540 2.58 - 2.64
Calcite 3 1.480 - 1.660 2.69 - 2.71
Chrysocolla 2 - 4 1.460 - 1.570 2.00 - 2.40
Kyanite 4 - 7 1.710 - 1.734 3.53 - 3.70
Citrine 7 1.544 - 1.553 2.65
Clinohumite 6 1.631 - 1.668 3.13 - 3.75
Carnelian 6.5 - 7 1.530 - 1.540 2.58 - 2.64
Chrysoberyl 8.5 1.746 - 1.763 3.70 - 3.78
Chrysoprase 6.5 - 7 1.530 - 1.540 2.58 - 2.64
Danburite 7 - 7.5 1.630 - 1.636 2.97 - 3.03
Demantoid 6.5 - 7 1.888 3.70 - 4.10
Diamond 10 2.417 - 2.419 3.50 - 3.53
Diaspro 6.5 - 7 1.54 2.58 - 2.91
Diopside 5 - 6 1.664 - 1.730 3.22 - 3.38
Heliodor 7.5 - 8 1.562 - 1.602 2.66 - 2.87
Heliotrope 6.5 - 7 1.530 - 1.540 2.58 - 2.64
Hematite 5.5 - 6.5 2.940 - 3.220 5.12 - 5.28
Epidote 6 - 7 1.729 - 1.768 3.30 - 3.50
Fluorite 4 1.434 3.00 - 3.25
Gahnite 7.5 - 8 1.712 - 1.762 4.00 - 4.62
Jade 6.5 - 7 1.652 - 1.688 3.30 - 3.38
Goshenite 7.5 - 8 1.562 - 1.602 2.66 - 2.87
Color Change Garnet 7 - 7.5 1.79 3.62 - 4.18
Malaia Garnet 7 - 7.5 1.74 3.75 - 3.99
Mali Garnet 7 - 7.5 1.730 - 1.890 3.42 - 4.10
Mandarin Garnet 7 - 7.5 1.81 4.12 - 4.18
Merelani Mint Garnet 7 - 7.5 1.74 3.57 - 3.73
Mozambique Garnet 7 - 7.5 1.790 - 1.830 3.62 - 3.87
Hessonite 6.5 - 7.5 1.734 - 1.759 3.57 - 3.73
Hiddenite 6.5 - 7 1.660 - 1.681 3.15 - 3.21
Howlite 3 - 3.5 1.586 - 1.609 2.45 - 2.58
Indicolite 7 - 7.5 1.614 - 1.666 2.82 - 3.32
Iolite 7 - 7.5 1.542 - 1.578 2.58 - 2.66
Kornerupine 6.5 - 7 1.660 - 1.699 3.27 - 3.45
Kunzite 6.5 - 7 1.660 - 1.681 3.15 - 3.21
Labradorite 6 - 6.5 1.559 - 1.570 2.65 - 2.75
Lapis lazuli 5 - 6 1.500 or 1.670 2.50 - 3.00
Malachite 3.5 - 4 1.655 - 1.909 3.25 - 4.10
Marcasite 6 - 6.5 N/A 5.00 - 5.20
Moldavite 5.5 1.480 - 1.540 2.32 - 2.38
Mookite 6.5 - 7 1.54 2.58 - 2.91
Morganite 7.5 - 8 1.562 - 1.602 2.71 - 2.90
Tiger’s Eye 6.5 - 7 1.534 - 1.540 2.58 - 2.64
Obsidian 5 - 5.5 1.450 - 1.550 2.35 - 2.60
Onyx 3.5 - 4 1.486 - 1.686 2.72 - 2.85
Opal 5.5 - 6.5 1.370 - 1.520 1.98 - 2.50
Ortoclase 6 - 6.5 1.518 - 1.526 2.56 - 2.59
Peridot 6.5 - 7 1.650 - 1.703 3.28 - 3.48
Pearl 2.5 - 4.5 1.520 - 1.660 2.60 - 2.85
Mother of Pearl 2.5 - 4.5 1.520 - 1.660 2.60 - 2.85
Petalite 6 - 6.5 1.502 - 1.519 2.4
Pezzottaite 7.5 - 8 1.562 - 1.602 2.66 - 2.87
Moonstone 6 - 6.5 1.518 - 1.526 2.56 - 2.59
Sunstone 6 - 6.5 1.510 - 1.550 2.62 - 2.65
Piropo 7 - 7.5 1.746 3.51 - 3.65
Prehnite 6 - 6.5 1.611 - 1.669 2.82 - 2.94
Quartz 7 1.544 - 1.553 2.64 - 2.69
Quarzite 7 1.544 - 1.553 2.64 - 2.69
Rhodocrosite 4 1.600 - 1.820 3.45 - 3.70
Rhodolite 7 - 7.5 1.76 3.62 - 3.87
Rhodonite 5.5 - 6.5 1.716 - 1.752 3.40 - 3.74
Pink Quartz 7 1.544 - 1.553 2.65
Rubellite 7 - 7.5 1.624 - 1.644 2.82 - 3.32
Ruby 9 1.762 - 1.778 3.97 - 4.05
Sapphire 9 1.762 - 1.788 3.95 - 4.03
Sard 6.5 - 7 1.530 - 1.540 2.58 - 2.64
Sardonyx 6.5 - 7 1.530 - 1.540 2.60 - 2.64
Scapolite 5.5 - 6 1.540 - 1.579 2.57 - 2.74
Scheelite 4.5 - 5 1.918 - 1.937 5.9 - 6.3
Sphene 5 - 5.5 1.843 - 2.110 3.52 - 3.54
Silimanite 6 - 7 1.650 - 1.680 3.24
Emerald 7.5 - 8 1.565 - 1.602 2.67 - 2.78
Sodalite 5.5 - 6 1.48 2.14 - 2.40
Spectrolite 6 - 6.5 1.559 - 1.570 2.65 - 2.75
Spessartite 7 - 7.5 1.81 4.12 - 4.18
Spinel 8 1.712 - 1.762 3.54 - 3.63
Sugilite 6 - 6.5 1.607 - 1.611 2.76 - 2.80
Tanzanite 6.5 - 7 1.691 - 1.700 3.35
Topaz 8 1.609 - 1.643 3.49 - 3.57
Tourmaline 7 - 7.5 1.624 - 1.644 2.82 - 3.32
Cuprian Tourmaline 7 - 7.5 1.614 - 1.666 2.82 - 3.32
Tsavorite 7 - 7.5 1.74 3.57 - 3.73
Turquoise 5 - 6 1.610 - 1.650 2.31 - 2.84
Paraiba Tourmaline 7 - 7.5 1.618 - 1.646 2.82 - 3.32
Umbalite 7 - 7.5 1.76 3.62 - 3.87
Unakite 6 - 7 1.525 - 1.760 2.85 - 3.20
Vesuvianite 6.5 1.700 - 1.723 3.32 - 3.47
Zircon 6.5 - 7.5 1.810 - 2.024 3.93 - 4.73
Zultanite 6.5 - 7 1.700 - 1.750 3.30 - 3.39