One of the most important and distinguishing qualities of a gem stone
is the property of enduring, resisting wear,—in short, hardness.
To test the hardness of precious stones that have not been cut or
polished, the following scale of ten minerals has been devised by Moh,
a German mineralogist:
No. 1. Talc. Very soft; is easily broken or scratched
with the finger-nail.
No. 2. Rock-salt. Soft; scratched with difficulty with
finger-nail; readily cut with a knife.
No. 3. Calcite. Low degree of hardness; not to be
scratched with finger-nail; easily scratched with a
knife.
No. 4. Fluor spar. Fairly hard; is slightly scratched
by a knife, but easily attacked with a file.
No. 5. Apatite. Medium hardness; does not scratch
glass, or only faintly; does not give out sparks
against steel; easily attacked with a file.
No. 6. Felspar. Easily scratches glass; is attacked by
a file, and gives some sparks against steel.
No. 7. Quartz. Quite hard; is only slightly attacked by
file; gives sparks readily against steel.
No. 8. Topaz. Very hard; is not attacked by a file.
No. 9. Sapphire. Hardest of all minerals but the
diamond; attacks all other minerals.
No. 10. Diamond. Attacks all minerals; is not attacked
by any.
To find the hardness of a stone, begin to test with the softest
mineral, so that when the number is reached which will scratch the
stone, there has been no injury to the specimen under examination. Half
numbers are determined by the ease or difficulty with which a stone is
scratched. For example, a stone which will resist No. 7 (quartz) and
which is only faintly attacked by No. 8 (topaz) may be safely put down
as 7.5, while a stone which resisted No. 7 and yielded easily to No. 8
is to be classed as 7 in hardness.
These tests are readily applied to crystals or unpolished gems. With
the polished stone greater care must be observed, and while a file
test is often satisfactory, there is always the danger of striking the
cleavage and breaking off a small piece of the stone.