Vital Statistics
There are three important measurements of a
gem's physical and optical properties you should
be aware of:
1. 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
natural substance is Diamond, followed by
Corundum (Ruby and Sapphire) then Topaz.
2. REFRACTIVE INDEX: Critical in gem
identification, the refractive index measures a
gemstone's brilliance. Developed by Dutch
mathematician Willebrord Snellius (Willebrord
Snel van Royen) in the 17th century, we
determine a gem's refractive index by using a refractometer to measure the speed and angle of
light as it enters the gem.
3. RELATIVE DENSITY: Due to
differences in their chemical composition and
crystal structure some gem types are heavier
than others. Relative density (grammes per cubic
centimetre 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. Relative density is
what jewellery appraisers use to estimate the
weight of gemstones without removing them from
their settings. They simply measure the set gem
with callipers and then extrapolate an estimate.
While you can find a gem's exact weight by
removing it from its mounting and weighing the
gem on a jeweller's scale, this should generally
be avoided. Doing so will expose both the gem
and the mounting to increased levels of wear and
tear.
 |
|
Gemstone |
Hardness |
Refractive Index |
Relative Density |
| 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 |
1.830 |
4.30 |
| Amazonite |
6 - 6.5 |
1.522 - 1.530 |
2.56 - 2.58 |
| Amber |
2 - 2.5 |
1.539 - 1.545 |
1.05 - 1.09 |
| Amblygonite |
6 |
1.578 - 1.646 |
3.01 - 3.11 |
| 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 |
| Aquamarine |
7.5 - 8 |
1.564 - 1.596 |
2.68 - 2.74 |
| 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 |
| Bixbite |
7.5 - 8 |
1.562 - 1.602 |
2.66 - 2.87 |
| Bloodstone |
6.5 - 7 |
1.530 - 1.540 |
2.58 - 2.64 |
| Bronzite |
5.5 |
1.650 - 1.680 |
3.20 - 3.30 |
| Calcite |
3 |
1.480 - 1.660 |
2.69 - 2.71 |
| Carnelian |
6.5 - 7 |
1.530 - 1.540 |
2.58 - 2.64 |
| Chalcedony |
6.5 - 7 |
1.530 - 1.540 |
2.58 - 2.64 |
| Chrysoberyl |
8.5 |
1.746 - 1.763 |
3.70 - 3.78 |
| Chrysocolla |
2 - 4 |
1.460 - 1.570 |
2.00 - 2.40 |
| Chrysoprase |
6.5 - 7 |
1.530 - 1.540 |
2.58 - 2.64 |
| Citrine |
7 |
1.544 - 1.553 |
2.65 |
| Clinohumite |
6 |
1.631 - 1.668 |
3.13 - 3.75 |
| Colour Change Garnet |
7 - 7.5 |
1.790 |
3.62 - 4.18 |
| Cuprian Tourmaline |
7 - 7.5 |
1.614 - 1.666 |
2.82 - 3.32 |
| 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 |
| Diopside |
5 - 6 |
1.664 - 1.730 |
3.22 - 3.38 |
| Emerald |
7.5 - 8 |
1.565 - 1.602 |
2.67 - 2.78 |
| 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 |
| Goshenite |
7.5 - 8 |
1.562 - 1.602 |
2.66 - 2.87 |
| Heliodor |
7.5 - 8 |
1.562 - 1.602 |
2.66 - 2.87 |
| Hematite |
5.5 - 6.5 |
2.940 - 3.220 |
5.12 - 5.28 |
| 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 |
| Idocrase |
6.5 |
1.700 - 1.723 |
3.32 - 3.47 |
| Indicolite |
7 - 7.5 |
1.614 - 1.666 |
2.82 - 3.32 |
| Iolite |
7 - 7.5 |
1.542 - 1.578 |
2.58 - 2.66 |
| Jade |
6.5 - 7 |
1.652 - 1.688 |
3.30 - 3.38 |
| Jasper |
6.5 - 7 |
1.540 |
2.58 - 2.91 |
| Kornerupine |
6.5 - 7 |
1.660 - 1.699 |
3.27 - 3.45 |
| Kunzite |
6.5 - 7 |
1.660 - 1.681 |
3.15 - 3.21 |
| Kyanite |
4 - 7 |
1.710 - 1.734 |
3.53 - 3.70 |
| 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 |
| Malaia Garnet |
7 - 7.5 |
1.740 |
3.75 - 3.99 |
| Mali Garnet |
7 - 7.5 |
1.730 - 1.890 |
3.42 - 4.10 |
| Mandarin Garnet |
7 - 7.5 |
1.810 |
4.12 - 4.18 |
| Marcasite |
6 - 6.5 |
N/A |
5.00 - 5.20 |
| Merelani Mint Garnet |
7 - 7.5 |
1.740 |
3.57 - 3.73 |
| Moldavite |
5.5 |
1.480 - 1.540 |
2.32 - 2.38 |
| Mookite |
6.5 - 7 |
1.540 |
2.58 - 2.91 |
| Moonstone |
6 - 6.5 |
1.518 - 1.526 |
2.56 - 2.59 |
| Morganite |
7.5 - 8 |
1.562 - 1.602 |
2.71 - 2.90 |
| Mother of Pearl |
2.5 - 4.5 |
1.520 - 1.660 |
2.60 - 2.85 |
| Mozambique Garnet |
7 - 7.5 |
1.790 - 1.830 |
3.62 - 3.87 |
| 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 |
| Orthoclase |
6 - 6.5 |
1.518 - 1.526 |
2.56 - 2.59 |
| Paraíba Tourmaline |
7 - 7.5 |
1.618 - 1.646 |
2.82 - 3.32 |
| Pearl |
2.5 - 4.5 |
1.520 - 1.660 |
2.60 - 2.85 |
| Peridot |
6.5 - 7 |
1.650 - 1.703 |
3.28 - 3.48 |
| Petalite |
6 - 6.5 |
1.502 - 1.519 |
2.40 |
| Pezzottaite |
7.5 - 8 |
1.562 - 1.602 |
2.66 - 2.87 |
| Prehnite |
6 - 6.5 |
1.611 - 1.669 |
2.82 - 2.94 |
| Pyrope |
7 - 7.5 |
1.746 |
3.51 - 3.65 |
| Quartz |
7 |
1.544 - 1.553 |
2.64 - 2.69 |
| Quartzite |
7 |
1.544 - 1.553 |
2.64 - 2.69 |
| Rhodochrosite |
4 |
1.600 - 1.820 |
3.45 - 3.70 |
| Rhodolite |
7 - 7.5 |
1.760 |
3.62 - 3.87 |
| Rhodonite |
5.5 - 6.5 |
1.716 - 1.752 |
3.40 - 3.74 |
| Rose 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 |
| Sillimanite |
6 - 7 |
1.650 - 1.680 |
3.24 |
| Sodalite |
5.5 - 6 |
1.480 |
2.14 - 2.40 |
| Spectrolite |
6 - 6.5 |
1.559 - 1.570 |
2.65 - 2.75 |
| Spessartite |
7 - 7.5 |
1.810 |
4.12 - 4.18 |
| Sphene |
5 - 5.5 |
1.843 - 2.110 |
3.52 - 3.54 |
| Spinel |
8 |
1.712 - 1.762 |
3.54 - 3.63 |
| Sugilite |
6 - 6.5 |
1.607 - 1.611 |
2.76 - 2.80 |
| Sunstone |
6 - 6.5 |
1.510 - 1.550 |
2.62 - 2.65 |
| Tanzanite |
6.5 - 7 |
1.691 - 1.700 |
3.35 |
| Tiger's Eye |
6.5 - 7 |
1.534 - 1.540 |
2.58 - 2.64 |
| Topaz |
8 |
1.609 - 1.643 |
3.49 - 3.57 |
| Tourmaline |
7 - 7.5 |
1.624 - 1.644 |
2.82 - 3.32 |
| Tsavorite |
7 - 7.5 |
1.740 |
3.57 - 3.73 |
| Turquoise |
5 - 6 |
1.610 - 1.650 |
2.31 - 2.84 |
| Umbalite |
7 - 7.5 |
1.760 |
3.62 - 3.87 |
| Unakite |
6 - 7 |
1.525 - 1.760 |
2.85 - 3.20 |
| Yellow Beryl |
7.5 - 8 |
1.562 - 1.602 |
2.66 - 2.87 |
| 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 |
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