ne crystallization.)

4. A small quartz crystal. (This may be either amethyst or quartz-topaz
or the common colorless variety. The fine, sharp, colorless crystals
from Herkimer County, N. Y., are excellent. These are very inexpensive.)

5. A fragment of a crystal of feldspar. (Common orthoclase feldspar,
which is frequently of a brownish pink or flesh color, will do.)

These five test stones represent the following degrees of hardness:

1. Carborundum is harder than any gem material but diamond. It will
scratch sapphire and ruby, which are rated 9 in hardness, hence we may
call carborundum 9-1/2 if we wish. It is, however, very much softer than
diamond, and the latter will scratch it upon the slightest pressure.

2. Sapphire, of hardness 9, scratching any gem material except diamond.

3. True topaz, of hardness 8. It is scratched by sapphire (and, of
course, ruby), also by chrysoberyl (which is hence rated 8-1/2), but
scratches most other stones. Spinel (which is also rated as 8 in
hardness) is really a bit harder than topaz.

4. Quartz, of hardness 7, and scratched by all the previous stones but
scratching those that were listed above as of less hardness than a file.

5. Feldspar, of hardness 6, hence slightly softer than a file and
yielding to it, but scratching the stones likewise rated as 6 when
applied forcibly to them. Also scratching stones rated as less than 6 on
slight pressure.

We must next consider how these minerals may be safely used upon gem
material. Obviously it would be far safer to use them upon rough gem
material than upon cut stones. However, with care and some little skill,
one may make hardness tests without particular danger to fine cut
material.

The way to proceed is to apply the cut stone (preferably its girdle, or
if that is so set as not to be available, a corner where several facets
meet) gently to the flat surface of one of the softer test stones,
drawing it lightly along the surface and noting the _feel_ and looking
to see if a scratch results. If the test stone is scratched try the next
harder test stone similarly. _Do not attempt to use the test stone upon
any valuable cut stone._ Proceed as above until the gem meets a test
stone that it does not attack. Its hardness is then probably equal to
the latter and perhaps if pressed forcibly against it a slight scratch
would result, but it is not advisable to resort to heavy pressure. A
light touch should be cultivated in this work.