G
Guest
I found the below book a fantastic read, it may be of help, I do hope you find it as interesting as I did. J.
Gem Hardness.
Every. possessor of a diamond ring is aware
that diamond easily scratches window-glass.
If other stones were tried, it would be found that
they also scratched glass, but not so readily, and,
if the experiment were extended, it would be found
that topaz scratches quartz, but is scratched by
corundum, which in its turn yields to the all powerful
diamond. There is therefore considerable
variation in the capacity of precious stones to
resist abrasion, or, as it is usually termed, in their
hardness. To simplify the mode of expressing this
character the mineralogist Mohs about a century
ago devised the following arbitrary scale, which is
still in general use.
MOHS'S SCALE OF HARDNESS
1. Talc
2. Gypsum
3. Calcite
4. Fluor
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond
a hardened file approaches quartz in hardness, and
easily scratches glass.
By saying that a stone has hardness 7 we merely
mean that it will not scratch quartz, and quartz
will not scratch it. The numbers indicate an order,
and have no quantitative significance whatever. This
is an important point about which mistakes are
often made. We must not, for instance, suppose
that diamond has twice the hardness of apatite.
As a matter of fact, the interval between diamond
and corundum is immensely greater than that
between the latter and talc, the softest of mineral
substances. Intermediate degrees of hardness
are expressed by fractions. The number 8 for
chrysoberyl means that it scratches topaz as easily
as it itself is scratched by corundum. Pyrope
garnet is slightly harder than quartz, and its
hardness is said therefore to be 7 .
Delicate tests show that the structure of all
crystallized substances is more or less grained, like
that of wood, and the hardness for the same stone
varies in different directions. Kyanite is unique
in this respect, since its hardness ranges from 5 to 7;
it can therefore be scratched by a knife in some
directions, but not in others. In most substances,
however, the range is so small as to be quite imperceptible.
Slight variation is also apparent in the
hardness of different specimens of the same species.
The diamonds from Borneo and New South Wales
are so distinctly harder than those from South
Africa and other localities that, when first discovered,
some difficulty was experienced in cutting them.
Again, lapidaries find that while Ceylon sapphires
are harder than rubies, Kashmir sapphires are softer.
Hardness is a character of fundamental importance
in a stone intended for ornamental wear, since upon
it depends the durability of the polish and brilliancy.
Ordinary dust is largely composed of grains of
sand, which is quartz in a minute form, and a
gem-stone should therefore be at least as hard as
that. Paste imitations are little harder than 5, and
consequently, as experience shows, their polish does
not survive a few weeks' wear. Hardness is,
however, of little use as a discriminative test except
for distinguishing between topaz or harder stone and
paste. Diamond is so much harder than other
stones that it will leave a cut in glass quite different
from the scratch of even corundum. Paste, being so
soft, readily yields to the file, and is thus easily
distinguished from genuine stones. In applying the
test to a cut stone, it is best to remove it from its
mount and try the effect on the girdle, because
any scratch would be concealed afterwards by the
setting. Any mark should be rubbed with the
finger to assure that it is not due to powder from
the scratching agent ; confusion may often be caused
in this way when the two substances are of nearly
the same hardness.
It must not be overlooked that extreme hardness
is compatible with cleavability in certain directions
intimately connected with the crystalline structure ;
the property, in fact, characterizes many mineral
species of different degrees of hardness. Diamond
can be split in four directions parallel to the faces of
the regular octahedron, a property utilized by the
lapidary for shaping a stone previous to cutting it.
Topaz cleaves with considerable ease at right angles
to the principal crystallographic axis. Felspar has
two directions of cleavage nearly at right angles to
one another. The new gem-stone, kunzite, needs
cautious handling owing to the facility with which
it splits in two directions mutually inclined at
about 70.
All stones are more or less brittle, and will be
fractured by a sufficiently violent blow, but the
irregular surface of a fracture cannot be mistaken
for the brilliant flat surface given by a cleavage.
The cleavage is by no means induced with equal
facility in the species mentioned above. A considerable
effort is required to split diamond, but in the
case of topaz or kunzite incipient cleavage in the
shape of flaws may be started if the stone be merely
dropped on to a hard floor.
Extract from the book
GEM-STONES AND THEIR DISTINCTIVE CHARACTERS
BY G. F. HERBERT SMITH
M.A., D.Sc.
OF THE BRITISH MUSEUM (NATURAL HISTORY)
Book link
https://ia600401.us.archive.org/34/items/gemstonestheirdi00smit/gemstonestheirdi00smit.pdf
Gem Hardness.
Every. possessor of a diamond ring is aware
that diamond easily scratches window-glass.
If other stones were tried, it would be found that
they also scratched glass, but not so readily, and,
if the experiment were extended, it would be found
that topaz scratches quartz, but is scratched by
corundum, which in its turn yields to the all powerful
diamond. There is therefore considerable
variation in the capacity of precious stones to
resist abrasion, or, as it is usually termed, in their
hardness. To simplify the mode of expressing this
character the mineralogist Mohs about a century
ago devised the following arbitrary scale, which is
still in general use.
MOHS'S SCALE OF HARDNESS
1. Talc
2. Gypsum
3. Calcite
4. Fluor
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond
a hardened file approaches quartz in hardness, and
easily scratches glass.
By saying that a stone has hardness 7 we merely
mean that it will not scratch quartz, and quartz
will not scratch it. The numbers indicate an order,
and have no quantitative significance whatever. This
is an important point about which mistakes are
often made. We must not, for instance, suppose
that diamond has twice the hardness of apatite.
As a matter of fact, the interval between diamond
and corundum is immensely greater than that
between the latter and talc, the softest of mineral
substances. Intermediate degrees of hardness
are expressed by fractions. The number 8 for
chrysoberyl means that it scratches topaz as easily
as it itself is scratched by corundum. Pyrope
garnet is slightly harder than quartz, and its
hardness is said therefore to be 7 .
Delicate tests show that the structure of all
crystallized substances is more or less grained, like
that of wood, and the hardness for the same stone
varies in different directions. Kyanite is unique
in this respect, since its hardness ranges from 5 to 7;
it can therefore be scratched by a knife in some
directions, but not in others. In most substances,
however, the range is so small as to be quite imperceptible.
Slight variation is also apparent in the
hardness of different specimens of the same species.
The diamonds from Borneo and New South Wales
are so distinctly harder than those from South
Africa and other localities that, when first discovered,
some difficulty was experienced in cutting them.
Again, lapidaries find that while Ceylon sapphires
are harder than rubies, Kashmir sapphires are softer.
Hardness is a character of fundamental importance
in a stone intended for ornamental wear, since upon
it depends the durability of the polish and brilliancy.
Ordinary dust is largely composed of grains of
sand, which is quartz in a minute form, and a
gem-stone should therefore be at least as hard as
that. Paste imitations are little harder than 5, and
consequently, as experience shows, their polish does
not survive a few weeks' wear. Hardness is,
however, of little use as a discriminative test except
for distinguishing between topaz or harder stone and
paste. Diamond is so much harder than other
stones that it will leave a cut in glass quite different
from the scratch of even corundum. Paste, being so
soft, readily yields to the file, and is thus easily
distinguished from genuine stones. In applying the
test to a cut stone, it is best to remove it from its
mount and try the effect on the girdle, because
any scratch would be concealed afterwards by the
setting. Any mark should be rubbed with the
finger to assure that it is not due to powder from
the scratching agent ; confusion may often be caused
in this way when the two substances are of nearly
the same hardness.
It must not be overlooked that extreme hardness
is compatible with cleavability in certain directions
intimately connected with the crystalline structure ;
the property, in fact, characterizes many mineral
species of different degrees of hardness. Diamond
can be split in four directions parallel to the faces of
the regular octahedron, a property utilized by the
lapidary for shaping a stone previous to cutting it.
Topaz cleaves with considerable ease at right angles
to the principal crystallographic axis. Felspar has
two directions of cleavage nearly at right angles to
one another. The new gem-stone, kunzite, needs
cautious handling owing to the facility with which
it splits in two directions mutually inclined at
about 70.
All stones are more or less brittle, and will be
fractured by a sufficiently violent blow, but the
irregular surface of a fracture cannot be mistaken
for the brilliant flat surface given by a cleavage.
The cleavage is by no means induced with equal
facility in the species mentioned above. A considerable
effort is required to split diamond, but in the
case of topaz or kunzite incipient cleavage in the
shape of flaws may be started if the stone be merely
dropped on to a hard floor.
Extract from the book
GEM-STONES AND THEIR DISTINCTIVE CHARACTERS
BY G. F. HERBERT SMITH
M.A., D.Sc.
OF THE BRITISH MUSEUM (NATURAL HISTORY)
Book link
https://ia600401.us.archive.org/34/items/gemstonestheirdi00smit/gemstonestheirdi00smit.pdf
