<h2>CHAPTER VII.</h2>
<h3>PHYSICAL PROPERTIES.</h3>
<h4><span class="smcap">E—Hardness.</span></h4>
<p>Hardness is perhaps one of the most important features in a stone,
especially those of the "gem" series, for no matter how colour, lustre,
general beauty and even rarity may entitle a stone to the designation
"precious," unless it possesses great hardness it cannot be used as a
gem or jewel.</p>
<p>Consequently, the hardness of jewels is a matter of no small importance,
and by dint of indefatigable research, in tests and comparison, all
known precious stones have been classified in various scales or degrees
of hardness. The most popular and reliable table is that of Mohs, in
which he takes talc as the softest of the rarer minerals and classes
this as No. 1; from that he goes by gradual steps to the diamond, the
hardest of the stones, which he calls No. 10, and between these two all
other gems are placed. Here is given a complete list of Mohs's
arrangement of stones, according to their hardness, beginning at No. 1,
thus:<span class="pagenum"><SPAN name="Page_40" id="Page_40">[Pg 40]</SPAN></span>—</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Talc</td><td align='left'> 1</td></tr>
<tr><td align='left'>Rock salt</td><td align='left'> 2</td></tr>
<tr><td align='left'>Amber</td><td align='left'> 2-1/2</td></tr>
<tr><td align='left'>Calcite</td><td align='left'> 3</td></tr>
<tr><td align='left'>Malachite</td><td align='left'> 3-1/2</td></tr>
<tr><td align='left'>Jet</td><td align='left'> 3-1/2</td></tr>
<tr><td align='left'>Fluorspar</td><td align='left'> 4</td></tr>
<tr><td align='left'>Apatite</td><td align='left'> 5</td></tr>
<tr><td align='left'>Dioptase</td><td align='left'> 5</td></tr>
<tr><td align='left'>Kyanite (various)</td><td align='left'> 5-7</td></tr>
<tr><td align='left'>Haüynite</td><td align='left'> 5-1/2</td></tr>
<tr><td align='left'>Hæmatite</td><td align='left'> 5-1/2</td></tr>
<tr><td align='left'>Lapis lazuli</td><td align='left'> 5-1/2</td></tr>
<tr><td align='left'>Moldavite (various)</td><td align='left'> 5-1/2-6-1/2</td></tr>
<tr><td align='left'>Rhodonite</td><td align='left'> 5-1/2-6-1/2</td></tr>
<tr><td align='left'>Obsidian</td><td align='left'> 5-1/2</td></tr>
<tr><td align='left'>Sphene</td><td align='left'> 5-1/2</td></tr>
<tr><td align='left'>Opal (various)</td><td align='left'> 5-1/2-6-1/2</td></tr>
<tr><td align='left'>Nephrite</td><td align='left'> 5-3/4</td></tr>
<tr><td align='left'>Chrysolite</td><td align='left'> 6-7</td></tr>
<tr><td align='left'>Felspar</td><td align='left'> 6</td></tr>
<tr><td align='left'>Adularia </td><td align='left'>6</td></tr>
<tr><td align='left'>Amazon stone</td><td align='left'> 6</td></tr>
<tr><td align='left'>Diopside</td><td align='left'> 6</td></tr>
<tr><td align='left'>Iron pyrites</td><td align='left'> 6</td></tr>
<tr><td align='left'>Labradorite</td><td align='left'> 6</td></tr>
<tr><td align='left'>Turquoise</td><td align='left'> 6</td></tr>
<tr><td align='left'>Spodumene</td><td align='left'> 6-1/2-7</td></tr>
<tr><td align='left'>The Chalcedony group which embraces the Agate, Carnelian, etc.</td><td align='left'> 6-1/2</td></tr>
<tr><td align='left'>Demantoid</td><td align='left'> 6-1/2</td></tr>
<tr><td align='left'>Epidote</td><td align='left'> 6-1/2</td></tr>
<tr><td align='left'>Idocrase</td><td align='left'> 6-1/2</td></tr>
<tr><td align='left'>Garnets (see also "Red Garnets" below)</td><td align='left'> 6-1/2-7-1/2</td></tr>
<tr><td align='left'> Axinite</td><td align='left'> 6-3/4</td></tr>
<tr><td align='left'> Jadeite</td><td align='left'> 6-3/4</td></tr>
<tr><td align='left'> Quartz, including Rock-crystal,Amethyst, Jasper, Chrysoprase Citrine, etc.</td><td align='left'> 7</td></tr>
<tr><td align='left'>Jade</td><td align='left'> 7</td></tr>
<tr><td align='left'> Dichorite (water sapphire) </td><td align='left'>7-7-1/2</td></tr>
<tr><td align='left'> Cordierite</td><td align='left'> 7-1/4</td></tr>
<tr><td align='left'> Red Garnets (see also Garnets above)</td><td align='left'> 7-1/4</td></tr>
<tr><td align='left'> Tourmaline</td><td align='left'> 7-1/4</td></tr>
<tr><td align='left'> Andalusite</td><td align='left'> 7-1/2</td></tr>
<tr><td align='left'> Euclase</td><td align='left'> 7-1/2</td></tr>
<tr><td align='left'> Staurolite</td><td align='left'> 7-1/2</td></tr>
<tr><td align='left'> Zircon</td><td align='left'> 7-1/2</td></tr>
<tr><td align='left'> Emerald, Aquamarine, or Beryl</td><td align='left'> 7-3/4</td></tr>
<tr><td align='left'> Phenakite</td><td align='left'> 7-3/4</td></tr>
<tr><td align='left'> Spinel</td><td align='left'> 8</td></tr>
<tr><td align='left'> Topaz</td><td align='left'> 8</td></tr>
<tr><td align='left'> Chrysoberyl</td><td align='left'> 8-1/2</td></tr>
<tr><td align='left'> The Corundum group embracing the Ruby,Sapphire, etc.</td><td align='left'>9</td></tr>
<tr><td align='left'> Diamond</td><td align='left'>10</td></tr>
</table></div>
<p>(See also list of stones, arranged in their respective colours, in
Chapter XII.)</p>
<p>The method of testing is very simple. A representative selection of the
above stones, each with a sharp edge, is kept for the purpose of
scratching and being scratched, and those usually set apart for tests in
the various groups, are as follows:<span class="pagenum"><SPAN name="Page_41" id="Page_41">[Pg 41]</SPAN></span>—</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>1</td><td align='left'>Talc</td></tr>
<tr><td align='left'>2</td><td align='left'>Rock-salt, or Gypsum</td></tr>
<tr><td align='left'>3</td><td align='left'>Calcite</td></tr>
<tr><td align='left'>4</td><td align='left'>Fluorspar</td></tr>
<tr><td align='left'>5</td><td align='left'>Apatite</td></tr>
<tr><td align='left'>6</td><td align='left'>Felspar</td></tr>
<tr><td align='left'>7</td><td align='left'>Quartz</td></tr>
<tr><td align='left'>8</td><td align='left'>Topaz</td></tr>
<tr><td align='left'>9</td><td align='left'>Corundum</td></tr>
<tr><td align='left'>10</td><td align='left'>Diamond</td></tr>
</table></div>
<p>The stone under examination may perhaps first be somewhat roughly
classified by its colour, cleavage, and general shape. One of these
standard stones is then gently rubbed across its surface and then others
of increasingly higher degrees, till no scratch is evident under a
magnifying glass. Thus if quartz ceases to scratch it, but a topaz will
do so, the degree of hardness must lie between 7 and 8. Then we reverse
the process: the stone is passed over the standard, and if both quartz
and topaz are scratched, then the stone is at least equal in hardness to
the topaz, and its classification becomes an easy matter.</p>
<p>Instead of stones, some experts use variously-tempered needles of
different qualities and compositions of iron and steel. For instance, a
finely-tempered ordinary steel needle will cut up to No. 6 stones; one
made of tool steel, up to 7; one of manganese steel, to 7-1/2; one made
of high-speed tool steel, to 8 and 8-1/2, and so on, according to
temper; so that from the scratch which can be made with the finger-nail
on mica, to the hardness of the diamond, which diamond alone will
scratch readily, the stones may be picked out, classified and tested,
with unerring accuracy.</p>
<p>It will thus be seen how impossible it is, even in this one of many
tests, for an expert to be deceived in the purchase of precious stones,
except through gross carelessness—a<span class="pagenum"><SPAN name="Page_42" id="Page_42">[Pg 42]</SPAN></span> fault seldom, if ever, met with in
the trade. For example—a piece of rock-crystal, chemically coloured,
and cut to represent a ruby, might appear so like one as to deceive a
novice, but the mere application to its surface of a real ruby, which is
hardness 9, or a No. 9 needle, would reveal too deep or powdery a
scratch; also its possibility of being scratched by a topaz or a No. 8
needle, would alone prove it false, for the corundum group, being harder
than No. 8, could not be scratched by it. So would the expert go down
the scale, the tiny scratches becoming fainter as he descended, because
he would be approaching more nearly the hardness of the stone under
test, till he arrived at the felspar, No. 6, which would be too soft to
scratch it, yet the stone would scratch the felspar, but not zircon or
andalusite, 7-1/2, or topaz, 8, so that his tests would at once classify
the stone as a piece of cut and coloured quartz, thus confirming what he
would, at the first sight, have suspected it to be.</p>
<p>The standard stones themselves are much more certain in results than the
needles, which latter, though well selected and tempered, are not
altogether reliable, especially in the more delicate distinctions of
picking out the hardest of certain stones of the same kind, in which
cases only the expert judge can decide with exactness. Accurate in this
the expert always is, for he judges by the sound and depth of his cut,
and by the amount and quality of the powder, often calling the
microscope to his aid, so that when the decision is made finally, there
is never the least doubt about it.</p>
<p>Rapidly as these tests can be made, they are extremely reliable, and
should the stone be of great value, it is<span class="pagenum"><SPAN name="Page_43" id="Page_43">[Pg 43]</SPAN></span> also subjected to other
unerring tests of extreme severity, any one of which would prove it
false, if it chanced to be so, though some stones are manufactured and
coloured so cleverly that to all but the expert judge and experienced
dealer, they would pass well for the genuine.</p>
<p>In Mohs's list it will be seen that several stones vary considerably,
the opal, for instance, having a degree of hardness from 5-1/2 to 6-1/2
inclusive. All stones differ slightly, though almost all may be said to
fit their position in the scale; but in the case of the opal, the
difference shown is partly due to the many varieties of the stone, as
described in the last chapter.</p>
<p>In applying this test of hardness to a cut gem, it will be noticed that
some parts of the same stone seem to scratch more readily than others,
such as on a facet at the side, which is often softer than those nearest
the widest part of the stone, where the claws, which hold it in its
setting, usually come. This portion is called the "girdle," and it is on
these "girdle" facets that the scratches are generally made. This
variation in hardness is mostly caused by cleavage, these cleavage
planes showing a marked, though often but slight, difference in the
scratch, which difference is <i>felt</i> rather than seen. In addition to the
peculiar <i>feel</i> of a cutting scratch, is the <i>sound</i> of it. On a soft
stone being cut by a hard one, little or no sound is heard, but there
will form a plentiful supply of powder, which, on being brushed off,
reveals a more or less deep incision. But as the stones approach one
another in hardness, there will be little powder and a considerable
increase in the noise; for the harder are the stones, cutting and being
cut, the louder will be the<span class="pagenum"><SPAN name="Page_44" id="Page_44">[Pg 44]</SPAN></span> sound and the less the powder. An example
of this difference is evident in the cutting of ordinary glass with a
"set" or "glazier's" diamond, and with a nail. If the diamond is held
properly, there will be heard a curious sound like a keen, drawn-out
"kiss," the diamond being considerably harder than the material it cut.
An altogether different sound is that produced by the scratching of
glass with a nail. In this case, the relative difference in hardness
between the two is small, so that the glass can only be scratched and
not "cut" by the nail; it is too hard for that, so the noise is much
greater and becomes a screech. Experience, therefore, makes it possible
to tell to a trifle, at the first contact, of what the stone is
composed, and in which class it should be placed, by the mere "feel" of
the scratch, the depth of it, the amount and kind of powder it leaves,
and above all, by the sound made, which, even in the tiniest scratch, is
quite characteristic.</p>
<hr style="width: 65%;" />
<p><span class="pagenum"><SPAN name="Page_45" id="Page_45">[Pg 45]</SPAN></span></p>
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