Specific Gravity

Density of Minerals. Each kind of precious stone has its own density. That is, if pieces of different stones were taken all of the same size, the weights would differ, but like-sized pieces of one and the same material always have the same weight. It is the custom among scientists to compare the densities of substances with the density of water. The number which expresses the relation between the density of any substance and the density of water is called the specific gravity number of the substance. For example, if, size for size, a material, say diamond, is 3.51 times as heavy as water, its specific gravity is 3.51. It will be seen that since each substance always has, when pure, the same specific gravity, we have here a means of distinguishing precious stones. It is very seldom, if ever, the case that we find any two precious stones of the same specific gravity. A few stones have nearly the same specific gravities, and in such cases it is well to apply other tests as well. Always make sure of a stone by seeing that two or three different tests point to the same species.

If the shape of a stone were such that the volume could be readily calculated, then the weight could be easily compared with the volume or with the weight of the same volume of water, and thus get the specific gravity (for a specific gravity number really tells how much heavier a piece of material is than the same volume of water).

Unfortunately the form of most precious stones is such that it would be very difficult to calculate the volume from the measurements, and the latter would be hard to make accurately with small stones. To avoid these difficulties the following ingenious method has been devised:

If a stone is dropped into water it pushes aside, or displaces, a body of water exactly equal in volume to itself. If the water thus displaced were caught and weighed, and the weight of the stone then divided by the weight of the water displaced, we would have the specific gravity number of the stone.

This is precisely what is done in getting the specific gravity of small stones. To make sure of getting an accurate result for the weight of water displaced the following apparatus is used.

Figure 1: Specific Gravity Bottle

The Specific Gravity Bottle. A small flask-like bottle (see Figure 1) is obtained. This has a tightly fitting ground glass stopper (B). The stopper has a small hole (C) drilled through it lengthwise. If the bottle is filled with water, and the stopper dropped in and tightened, water will squirt out through the small hole in the stopper. On wiping off stopper and bottle we have the bottle exactly full of water. If now the stopper is removed, the stone to be tested (which must of course be smaller than the neck of the bottle) dropped in, and the stopper replaced, exactly as much water will squirt out as is equal in volume to the stone that was dropped in.

 

If we had weighed the full bottle with the stone on the pan beside it, and then weighed the bottle with the stone inside it we could now, by subtracting the last weight from the first, find out how much the water, that was displaced, weighed. This is precisely the thing to do. The weight of the stone being known we now have merely to divide the weight of the stone by the weight of the displaced water, and we have the specific gravity number. Reference to a table of specific gravities of precious stones will enable us to name our stone.

A Sample Calculation.   The test takes less time than it would to read this description for those skilled in the actual performance of the operation. The following sample calculation may help make the matter clearer:

 

 

Weight of bottle + stone (outside) = 53.51 carats
Weight of bottle + stone (inside) = 52.51 carats
Weight of water displaced = 1.00 carat
Weight of stone = 3.51 carats

 

 

Specific gravity = Weight of stone/ = 3.51/ = 3.51 SG
Weight of water 1.00

 

In this case the specific gravity being 3.51, the stone is probably diamond (see table), but might be precious topaz, which has nearly the same specific gravity.

It is assumed that the jeweller will weigh in carats, and that his balance is sensitive to .01 carat. With such a balance, and a specific gravity bottle, results sufficiently accurate for the determination of precious stones may be had as long as air bubbles are excluded from the bottle, and the outside of the bottle is perfectly dry before each weighing. The bottle should never be held in the warm hands, or it will act like a thermometer and expand the water up the narrow tube in the stopper, thus leading to error. A handkerchief may be used to grasp the bottle.

Table of Specific Gravities of the Principal Gem Materials

 

Beryl (Emerald) 2.74
Chrysoberyl (Alexandrite) 3.73
Corundum (Ruby, sapphire, "Oriental topaz")            4.03
Diamond 3.52
Garnet (Pyrope) 3.78
" (Hessonite) 3.61
" (Demantoid) 3.84
" (Almandite) 4.05
Opal 2.15
Peridot 3.40
Quartz 2.66
Spinel (Rubicelle, Balas ruby) 3.60
Spodumene (Kunzite) 3.18
Topaz (precious) 3.53
Tourmaline 3.10
Turquoise 2.82
Zircon,  lighter variety 4.20
" heavier variety 4.69


Weighing a Gem in Water.
  Sometimes gems will be too large for the neck of a specific gravity bottle. In this case we resort to another method of finding how much a like volume of water weighs.

Figure 2: Measuring SG of Large Stones

 If the stone, instead of being dropped into a perfectly full bottle of water (which then overflows), is dropped into a partly filled glass or small beaker of water, just as much water will be displaced as though the vessel were full, and it will be displaced upward as before, for lack of any other place to go. Consequently its weight will tend to buoy up or float the stone by trying to get back under it, and the stone when in water will weigh less than when in air. Anyone who has ever pulled up a small anchor when out fishing from a boat will recognize at once that this is the case, and that as the anchor emerges from the water it seems to suddenly grow heavier. Not only does the stone weigh less when in the water, but it weighs exactly as much less as the weight of the water that was displaced by the stone (which has a volume equal to the volume of the stone). If we weigh a stone first in the air, as usual, and then in water (where it weighs less), and then subtract the weight in water from the weight in air we will have the loss of weight in water, and this equals the weight of an equal volume of water, which is precisely what we got by our bottle method.

We now need only divide the weight in air by the loss of weight in water, and we have the specific gravity of the stone.

To actually weigh the stone in water we must use a fine wire to support the stone. We must first find how much this wire itself weighs (when attached by a small loop to the hook that supports the balance pan and trailing partly in the water, as will be the case when weighing the stone in water). This weight of the wire must of course be deducted to get the true weight of the stone in water. The beaker of water is best supported by a small table that stands over the balance pan (See Figure 2).

Weight of stone = 4.02 carats
Weight of stone (plus wire) in water = 3.32 carats
Weight of wire = .30 carat
True weight of stone in water = 3.02 carats
Loss of weight in water = 1.00 carat
Specific gravity = Weight of stone = 4.02 = 4.02
Loss in water 1.00

 

Here the specific gravity, 4.02 would indicate some corundum gem (ruby or sapphire), and the other characters would indicate at once which it was.

The specific gravity method is of special value in distinguishing between the various colorless stones, as, for example, quartz crystal, true white topaz, white sapphire, white or colorless beryl, etc. These are all doubly refractive, have no color, and hence no dichroism, and unless one has a refractometer to get the refractive index, they are difficult to distinguish. The specific gravities are very different, however, and readily serve to distinguish them. It should be added that the synthetic stones show the same specific gravities as their natural counterparts, so that this test does not serve to detect them.