Jewelers often use a small file for hardness testing. If the file cuts into the stone, a fine powder is produced, thus showing that the stone is softer than the file. The hardness of a jeweler’s file, which is finely cut and of hard steel, is about 6 1/2. It will mark feldspar but will not scratch quartz. Glass and pastes are marked by a file, their hardness varying from 4 1/2 to 5 1/2. For use in jewelry, stones should withstand this file test, but many well known varieties are softer, their other outstanding qualities overcoming this deficiency.
Tests on transparent and cut stones should be applied with discretion since a specimen may be easily disfigured by a deep file mark. It should therefore be carefully tested on an edge, preferably the setting edge. The sound made by filing and the color of the powder produced are also guides to hardness. But in general, this test is more useful with rough stones than with cut gems; there are other tests for distinguishing the latter without risking injury to the beauty and value of the specimen.
It must be added that some stones vary slightly in their degree of hardness. This will depend in a few instances on their place of occurrence, in others, on the direction in which they are tested, for instance, the hardness of diamonds varies slightly, the Borneo stones being harder than the South African stones. Different faces of the same stone may also vary in hardness, this feature being due to the internal molecular structure. An outstanding example is kyanite, a mineral whose hardness is 5 in one direction and 7 in another. But generally speaking, the limits of variation are very close to each other in most stones.
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