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HOW TO FIND THE SPECIFIC GRAVITY AND HARDNESS OF STONES
To
obtain the specific gravity of a stone, first thoroughly cleanse it,
removing all grease and air-bubbles. As the Kohl-busch pocket-scale is
in most general use among jewellers, it is presumed that it will be
used in the experiment. Weigh the stone, noting the exact weight to the
smallest fraction. Then loop one of the cords by which one of the pans
is suspended through the ring from which it hangs, so that it will hang
sideways without interfering with the balance of the scales. Take a
thin silken thread, double it, make a double loop at one end that will
hold the stone and suspend it from the ring to which the pan is
attached; or, better still, take a fine aluminum wire, bend it at one
end to an open stud spiral, to a hook at the other; put the stone to be
weighed in the spiral and hook the wire to the ring from which the pan
is suspended. Place under it a wide-mouthed vial of convenient size
(about two and a half inches high, with a half-inch opening)
containing distilled water at 6o° F., and, hanging the stone immersed
in this, weigh it again. Deduct the weight of the wire. Then divide the
first weight (in air) by the difference between the first and second
weights (second in water), and the result will be the specific gravity.
Be sure that the stone hangs free and is completely immersed when it is
weighed.
To find the hardness of a stone use a finely tempered steel
point upon an under facet near the girdle. This will bite with
growing difficulty until quartz is reached. The higher the
tone which comes from the scratching, the harder the stone.
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