In testing a gem for hardness the problem consists in finding out which of the above minerals is most nearly equal in hardness to the unknown stone. Any gem that was approximately equal in hardness to a true topaz (8) would also be said to be of hardness 8. Thus spinel is of about the same hardness as topaz and hence is usually rated as 8 in hardness. Similarly opal, moonstone, and turquoise are of about the same hardness as feldspar and are all rated 6. Frequently stones will be found that in hardness are between some two of Mohs's minerals. In that case we add one half to the number of the softer mineral; thus, peridot, benitoite, and jade (nephrite) are all softer than quartz (7) but harder than feldspar (6); hence we say they are 6-1/2 in hardness. Beryl (aquamarine and emerald), garnet (almandine), and zircon are rated 7-1/2 in hardness, being softer than true topaz but harder than quartz. A table of the hardness of most of the commonly known gem-stones follows this lesson. Having now an idea of what hardness means and how it is expressed, we must next inquire how one may make use of it in identifying unknown gems. HOW TO APPLY THE HARDNESS TEST. In the first place, it is necessary to caution the beginner against damaging a fine gem by attempting to test its hardness in any but the most careful manner. The time-honored file test is really a hardness test and serves nicely to distinguish genuine gems, of hardness 7 or above, from glass imitations. A well-hardened steel file is of not quite hardness 7, and glass of various types while varying somewhat averages between 5 and 6. Hence, glass imitations are easily attacked by a file. To make the file test use only a _very fine_ file and apply it with a light but firm pressure lengthwise along the girdle (edge) of the unset stone. If damage results it will then be almost unnoticeable. Learn to know the _feel_ of the file as it takes hold of a substance softer than itself. Also learn the _sound_. If applied to a hard stone a file will slip on it, as a skate slips on ice. It will not take hold as upon a softer substance. If the stone is set, press a sharp corner of a broken-ended file gently against a _back_ facet, preferably high up toward the girdle, where any damage will not be visible from the front, and move the file very slightly along the surface, noting by the _feel_ whether or not it takes hold and also looking with a lens to see if a scratch has been made. Do not mistake a line of steel, left on a slightly rough surface, for a true scratch. Frequently on an unpolished girdle of real gem material the file will leave a streak of steel. Similarly when using test minerals in accordance with what follows do not mistake a streak of powder from the yielding test material, for a true scratch in the material being tested. The safe way is to wipe the spot thus removing any powder. A true scratch will, of course, persist.