Moh's Hardness Scale
In 1822, a German Mineralogist, Friedrich Moh, developed a hardness scale. While not scientifically based on the true numerical hardness of gemstones, it is so practical in its application, it's still in use today by most Geologists and Gemologists.

Some things should be said first--some stones/gems are harder in one direction than the other. For instance, Calcite is easily scratched with a nail in one direction (Hardness 2), but difficult to scratch in another direction (Hardness 3). 

Some handy points of reference are: a fingernail, about 2 -2 1/2. A copper penny is about a Hardness of 3, a steel knife, a bit over 5 and glass about 5 1/2. Really good steel runs about 6 1/2.

Moh's Hardness Scale
Stone Name   Hardness
ALEXANDRITE

-

8.5
AMBER - 2.0-3.0
AMETHYST - 7.0
AMETRINE - 7.0
ANDALUZITE - 7.0-7.5
APATITE - 5.0
AQUAMARINE - 7.5-8.0
BERYL - 7.5-8.0
CALCITE - 3.0
CHALCEDONY - 7.0
CHRYSOBERYL - 8.5
CHRYSOPHRASE - 6.5-7.0
CITRINE - 7.0
DANBURITE - 7.0
DIOPSIDE - 5.5-6.0
EMERALD - 7.5-8.0
ENSTATITE - 5.0-6.0
FLUORITE - 4.0-4.5
GARNET - 7.0-7.5
IOLITE - 7.0-7.5
JADEITE - 6.5-7.0
KORNERUPINE - 6.5-7.0
KUNZITE - 6.0-7.0
KYANITE - 4.0-7.5
LABRODORITE - 6.0-6.5
MALACHITE - 3.5-4.0
MORGANITE - 7.5-8.0
MOON STONE - 6.0-6.5
NEPHRITE - 6.0-6.5
OPAL - 5.5-6.5
PADPARADSCHA - 9.0
PEARL - 3.0-4.0
PERIDOT - 6.5-7.0
PRASIOLITE - 7.0
QUARTZ - 7.0
RHODOCHROSITE - 4.0
RHODOLITE - 7.0-7.5
ROSE QUARTZ - 7.0
RUBY - 9.0
RUBELLITE - 7.0-7.5
RUTELLITE - 7.0
SAPPHIRE - 9.0
SCAPOLITE - 6.0-6.5
SILLIMINITE (FIBROLITE) - 6.0-7.5
SINHALITE - 6.5
SPESSARTITE - 7.0-7.5
SPHALERITE - 3.5-4.0
SPHENE - 5.0-5.5
SPINEL - 8.0
STAR RUBY - 9.0
SUN STONE - 6.0-6.5
TSAVORITE - 7.0-7.5
TANZANITE - 6.5-7.0
TOURMALINE - 7.0-7.5
TOPAZ - 8.0
ZIRCON - 6.0-7.5
Hardness Description
Hardness Key Stone Comments
10 Diamond Hardest mineral known to man
9 Corundum
(Sapphire/Ruby)
 Harder than Topaz
8 Topaz  Harder than Quartz
7 Quartz Scratches Glass
6 Microcline Suitable for gems
5 Apatite About the hardness of teeth
4 Fluorite Cannot be scratched with a nail
3 Calcite Difficult to scratch with a nail
2 Gypsum Easily scratched with nail
1 Talc Very soft, chalk-like