Specific Gravity in Precious Stones

The properties so far considered as serving to distinguish precious stones have all depended upon the behavior of the material toward light.

These properties were considered first because they afford, to those acquainted with their use, very rapid and sure means of classifying precious stones.

Density of Minerals. We will next consider an equally certain test, which, however, requires rather more time, apparatus, and skill to apply.

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 also. In fact one should always make sure of a stone by seeing that two or three different tests point to the same species.

We must next find out how to determine the specific gravity of a precious stone. If the shape of a stone were such that the volume could be readily calculated, then one could easily compare the weight 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.

The Specific Gravity Bottle. A small flask-like bottle (see Fig. 5) 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. Such a table follows this lesson.

A Sample Calculation. The actual performance of the operation, if one is skilled in weighing, takes less time than it would to read this description. At first one will be slow, and perhaps one should read and re-read this lesson, making sure that all the ideas are clear before trying to put them in practice.

A sample calculation may help make the matter clearer, so one is appended:

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/Weight of water = 3.51/1.00 = 3.51 Sp. g.

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.