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.