<h2>CHAPTER VIII.</h2>
<h3>PHYSICAL PROPERTIES.</h3>
<h4><span class="smcap">F—Specific Gravity.</span></h4>
<p>The fixing of the specific gravity of a stone also determines its group
position with regard to weight; its colour and other characteristics
defining the actual stone. This is a safe and very common method of
proving a stone, since its specific gravity does not vary more than a
point or so in different specimens of the same stone. There are several
ways of arriving at this, such as by weighing in balances in the usual
manner, by displacement, and by immersion in liquids the specific
gravity of which are known. Cork is of less specific gravity than water,
therefore it floats on the surface of that liquid, whereas iron, being
heavier, sinks. So that by changing the liquid to one lighter than cork,
the cork will sink in it as does iron in water; in the second instance,
if we change the liquid to one heavier than iron, the iron will float on
it as does cork on water, and exactly as an ordinary flat-iron will
float on quicksilver, bobbing up and down like a cork in a tumbler of
water. If, therefore, solutions of known but varying densities are
compounded, it is possible to tell almost to exactitude the specific
gravity of any stone dropped into them, by the position they assume.
Thus, if we take a solution of<span class="pagenum"><SPAN name="Page_46" id="Page_46">[Pg 46]</SPAN></span> pure methylene iodide, which has a
specific gravity of 3.2981, and into this drop a few stones selected
indiscriminately, the effect will be curious: first, some will sink
plump to the bottom like lead; second, some will fall so far quickly,
then remain for a considerable time fairly stationary; third, some will
sink very slowly; fourth, some will be partially immersed, that is, a
portion of their substance being above the surface of the liquid and a
portion covered by it; fifth, some will float on the surface without any
apparent immersion. In the first case, the stones will be much heavier
than 3.2981; in the second, the stones will be about 3.50; in the third
and fourth instances, the stones will be about the same specific gravity
as the liquid, whilst in the fifth, they will be much lighter, and thus
a rough but tolerably accurate isolation may be made.</p>
<p>On certain stones being extracted and placed in other liquids of lighter
or denser specific gravity, as the case may be, their proper
classification may easily be arrived at, and if the results are checked
by actual weight, in a specific gravity balance, they will be found to
be fairly accurate. The solution commonly used for the heaviest stones
is a mixture of nitrate of thallium and nitrate of silver. This double
nitrate has a specific gravity of 4.7963, therefore such a stone as
zircon, which is the heaviest known, will float in it. For use, the
mixture should be slightly warmed till it runs thin and clear; this is
necessary, because at 60° (taking this as ordinary atmospheric
temperature) it is a stiff mass. A lighter liquid is a mixture of iodide
of mercury in iodide of potassium, but this is such an extremely
corrosive and<span class="pagenum"><SPAN name="Page_47" id="Page_47">[Pg 47]</SPAN></span> dangerous mixture, that the more common solution is one
in which methylene iodide is saturated with a mixture of iodoform until
it shows a specific gravity of 3.601; and by using the methylene iodide
alone, in its pure state, it having a specific gravity of 3.2981, the
stones to that weight can be isolated, and by diluting this with
benzole, its weight can be brought down to that of the benzole itself,
as in the case of Sonstadt's solution. This solution, in full standard
strength, has a specific gravity of 3.1789, but may be weakened by the
addition of distilled water in varying proportions till the weight
becomes almost that of water.</p>
<p>Knowing the specific gravity of all stones, and dividing them into six
groups, by taking a series of standard solutions selected from one or
other of the above, and of known specific gravity, we can judge with
accuracy if any stone is what it is supposed to be, and classify it
correctly by its mere floating or sinking when placed in these liquids.
Beginning then with the pure double nitrate of silver and thallium, this
will isolate the stones of less specific gravity than 4.7963, and taking
the lighter solutions and standardising them, we may get seven solutions
which will isolate the stones as follows:—</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>A</td><td align='left'>shows the</td><td align='left'>stones which have</td><td align='left'>a specific gravity over</td><td align='left'>4.7963</td></tr>
<tr><td align='left'>B</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='left'>3.70</td><td align='center'>and under</td><td align='left'>4.7963</td></tr>
<tr><td align='left'>C</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='left'>3.50</td><td align='center'>"</td><td align='left'>3.70</td></tr>
<tr><td align='left'>D</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='left'>3.00</td><td align='center'>"</td><td align='left'>3.50</td></tr>
<tr><td align='left'>E</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='left'>2.50</td><td align='center'>"</td><td align='left'>3.00</td></tr>
<tr><td align='left'>F</td><td align='center'>"</td><td align='center'>"</td><td align='center'>"</td><td align='left'>2.00</td><td align='center'>"</td><td align='left'>2.50</td></tr>
<tr><td align='left'>G</td><td align='center'>"</td><td align='center'>"</td><td align='center'>—</td><td align='center'>—</td><td align='center'>under</td><td align='left'>2.00</td></tr>
</table></div>
<p><span class="pagenum"><SPAN name="Page_48" id="Page_48">[Pg 48]</SPAN></span></p>
<p>Therefore each liquid will isolate the stones in its own group by
compelling them to float on its surface; commencing with the heaviest
and giving to the groups the same letters as the liquids, it is seen
that—</p>
<p><i>Group</i> A.—Isolates gems with a specific gravity of 4.7963 and over
4.70; in this group is placed zircon, with a specific gravity of from
4.70 to 4.88.</p>
<p><i>Group</i> B.—Stones whose specific gravity lies between 3.70 and under
4.7963.</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Garnets,</td><td colspan="3">many varieties. See Group D below.</td></tr>
<tr><td align='left'>Almandine</td><td align='right'>4.11</td><td align='left'>and occasionally to</td><td align='left'>4.25</td></tr>
<tr><td align='left'>Ruby</td><td align='right'>4.073</td><td align='center'>"</td><td align='left'>4.080</td></tr>
<tr><td align='left'>Sapphire</td><td align='right'>4.049</td><td align='center'>"</td><td align='left'>4.060</td></tr>
<tr><td align='left'>Corundum</td><td align='right'>3.90</td><td align='center'>"</td><td align='left'>4.16</td></tr>
<tr><td align='left'>Cape Ruby</td><td align='right'>3.861</td></tr>
<tr><td align='left'>Demantoid</td><td align='right'>3.815</td></tr>
<tr><td align='left'>Staurolite</td><td align='right'>3.735</td></tr>
<tr><td align='left'>Malachite</td><td align='right'>3.710</td><td align='left'>and occasionally to</td><td align='left'>3.996</td></tr>
</table></div>
<p><i>Group</i> C.—Stones whose specific gravity lies between 3.50 and under
3.70.</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Pyrope (average)</td><td align='left'>3.682</td></tr>
<tr><td align='left'>Chrysoberyl</td><td align='left'>3.689</td><td align='left'>and occasionally</td><td align='left'>to 3.752</td></tr>
<tr><td align='left'>Spinel</td><td align='left'>3.614</td><td align='center'>"</td><td align='left'>3.654</td></tr>
<tr><td align='left'>Kyanite</td><td align='left'>3.609</td><td align='center'>"</td><td align='left'>3.688</td></tr>
<tr><td align='left'>Hessonite</td><td align='left'>3.603</td><td align='center'>"</td><td align='left'>3.651</td></tr>
<tr><td align='left'>Diamond</td><td align='left'>3.502</td><td align='center'>"</td><td align='left'>3.564</td></tr>
<tr><td align='left'>Topaz</td><td align='left'>3.500</td><td align='center'>"</td><td align='left'>3.520</td></tr>
</table></div>
<p><i>Group</i> D.—Stones whose specific gravity lies between 3 and under 3.50.</p>
<p><span class="pagenum"><SPAN name="Page_49" id="Page_49">[Pg 49]</SPAN></span></p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Rhodonite</td><td align='left'>3.413</td><td align='left'>and occasionally to</td><td align='left'>3.617</td></tr>
<tr><td align='left'>Garnets</td><td align='left'>3.400</td><td align='center'>"</td><td align='left'>4.500</td></tr>
<tr><td align='left'>Epidote</td><td align='left'>3.360</td><td align='center'>"</td><td align='left'>3.480</td></tr>
<tr><td align='left'>Sphene</td><td align='left'>3.348</td><td align='left'>and occasionally to</td><td align='left'>3.420</td></tr>
<tr><td align='left'>Idocrase</td><td align='left'>3.346</td><td align='center'>"</td><td align='left'>3.410</td></tr>
<tr><td align='left'>Olivine</td><td align='left'>3.334</td><td align='center'>"</td><td align='left'>3.368</td></tr>
<tr><td align='left'>Chrysolite</td><td align='left'>3.316</td><td align='center'>"</td><td align='left'>3.528</td></tr>
<tr><td align='left'>Jade</td><td align='left'>3.300</td><td align='center'>"</td><td align='left'>3.381</td></tr>
<tr><td align='left'>Jadeite</td><td align='left'>3.299</td></tr>
<tr><td align='left'>Axinite</td><td align='left'>3.295</td></tr>
<tr><td align='left'>Dioptase</td><td align='left'>3.289</td></tr>
<tr><td align='left'>Diopside</td><td align='left'>2.279</td></tr>
<tr><td align='left'>Tourmaline (yellow)</td><td align='left'>3.210</td></tr>
<tr><td align='left'>Andalusite</td><td align='left'>3.204</td></tr>
<tr><td align='left'>Apatite</td><td align='left'>3.190</td></tr>
<tr><td align='left'>Tourmaline (Blue and Violet)</td><td align='left'>3.160</td></tr>
<tr><td align='left'>Tourmaline (Green)</td><td align='left'>3.148</td></tr>
<tr><td align='left'> " (Red)</td><td align='left'>3.100</td></tr>
<tr><td align='left'>Spodumene</td><td align='left'>3.130</td><td align='left'>and occasionally to</td><td align='left'>3.200</td></tr>
<tr><td align='left'>Euclase</td><td align='left'>3.090</td></tr>
<tr><td align='left'>Fluorspar</td><td align='left'>3.031</td><td align='left'>and occasionally to</td><td align='left'>3.200</td></tr>
<tr><td align='left'>Tourmaline (Colourless)</td><td align='left'>3.029</td></tr>
<tr><td align='left'>Tourmaline (Blush Rose)</td><td align='left'>3.024</td></tr>
<tr><td align='left'>Tourmaline (Black)</td><td align='left'>3.024</td><td align='left'>and occasionally to</td><td align='left'>3.300</td></tr>
<tr><td align='left'>Nephrite</td><td align='left'>3.019</td></tr>
</table></div>
<p><i>Group</i> E.—Stones whose specific gravity lies between 2.50 and under
3.000.</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Phenakite</td><td align='left'>2.965</td></tr>
<tr><td align='left'>Turquoise</td><td align='left'>2.800</td></tr>
<tr><td align='left'>Beryl</td><td align='left'>2.709</td><td align='left'> and occasionally to</td><td align='left'> 2.81</td></tr>
<tr><td align='left'>Aquamarine</td><td align='left'>2.701</td><td align='center'>"</td><td align='left'>2.80</td></tr>
<tr><td align='left'>Labradorite</td><td align='left'>2.700</td></tr>
<tr><td align='left'>Emerald</td><td align='left'>2.690</td></tr>
<tr><td align='left'>Quartz</td><td align='left'>2.670</td></tr>
<tr><td align='left'>Chrysoprase</td><td align='left'>2.670</td></tr>
<tr><td align='left'>Jasper</td><td align='left'>2.668</td></tr>
<tr><td align='left'>Amethyst</td><td align='left'>2.661</td></tr>
<tr><td align='left'>Hornstone</td><td align='left'>2.658</td></tr>
<tr><td align='left'>Citrine</td><td align='left'>2.658</td></tr>
<tr><td align='left'>Cordierite</td><td align='left'>2.641</td></tr>
<tr><td align='left'>Agate</td><td align='left'>2.610</td></tr>
<tr><td align='left'>Chalcedony</td><td align='left'>2.598</td><td align='left'> and occasionally to</td><td align='left'> 2.610</td></tr>
<tr><td align='left'>Adularia</td><td align='left'>2.567</td></tr>
<tr><td align='left'>Rock-crystal</td><td align='left'>2.521</td><td align='left'> and occasionally to</td><td align='left'> 2.795</td></tr>
</table></div>
<p><span class="pagenum"><SPAN name="Page_50" id="Page_50">[Pg 50]</SPAN></span></p>
<p><i>Group</i> F.—Stones whose specific gravity lies between 2.00 and under
2.50.</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Haüynite</td><td align='left'>2.470</td><td align='left'>and occasionally to</td><td align='left'>2.491</td></tr>
<tr><td align='left'>Lapis lazuli</td><td align='left'>2.461</td></tr>
<tr><td align='left'>Moldavite</td><td align='left'>2.354</td></tr>
<tr><td align='left'>Opal</td><td align='left'>2.160</td><td align='left'>and according to variety to</td><td align='left'>2.283</td></tr>
<tr><td align='left'> " (Fire Opal)</td><td align='left'>2.210</td><td align='left'>(average)</td></tr>
</table></div>
<p><i>Group</i> G.—Stones whose specific gravity is under 2.00.</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td align='left'>Jet</td><td align='left'>1.348</td></tr>
<tr><td align='left'>Amber</td><td align='left'>1.000</td></tr>
</table></div>
<div class="blockquot"><p>(See also list of stones, arranged in their respective colours,
in Chapter XII.)</p>
</div>
<p>In many of these cases the specific gravity varies from .11 to .20, but
the above are the average figures obtained from a number of samples
specially and separately weighed. In some instances this difference may
cause a slight overlapping of the groups, as in group C, where the
chrysoberyl may weigh from 3.689 to 3.752, thus bringing the heavier
varieties of the stone into group B, but in all cases where overlapping
occurs, the colour, form, and the self-evident character of the stone
are in themselves sufficient for classification, the specific gravity
proving genuineness. This is especially appreciated when<span class="pagenum"><SPAN name="Page_51" id="Page_51">[Pg 51]</SPAN></span> it is
remembered that so far science has been unable (except in very rare
instances of no importance) to manufacture any stone of the same colour
as the genuine and at the same time of the same specific gravity. Either
the colour and characteristics suffer in obtaining the required weight
or density, or if the colour and other properties of an artificial stone
are made closely to resemble the real, then the specific gravity is so
greatly different, either more or less, as at once to stamp the jewel as
false. In the very few exceptions where chemically-made gems even
approach the real in hardness, colour, specific gravity, &c., they cost
so much to obtain and the difficulties of production are so great that
they become mere chemical curiosities, far more costly than the real
gems. Further, they are so much subject to chemical action, and are so
susceptible to their surroundings, that their purity and stability
cannot be maintained for long even if kept airtight; consequently these
ultra-perfect "imitations" are of no commercial value whatever as
jewels, even though they may successfully withstand two or three tests.</p>
<hr style="width: 65%;" />
<p><span class="pagenum"><SPAN name="Page_52" id="Page_52">[Pg 52]</SPAN></span></p>
<div style="break-after:column;"></div><br />
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