Mineral Guide > Gems > Specific Gravity

Specific Gravity

The specific gravity of mineral species is also one of their fundamental and constant characters, and furnishes a reliable means of distinguishing between gems of different kinds, and of separating false from real stones. To be sure, a variation of composition may cause a variation of specific gravity in the same species ; but this is usually within comparatively narrow limits.

The different kinds of garnet, or of tourmaline, for example, possess specific gravities varying within one integer; but the varieties are usually distinguished by colors by which the appropriate specific gravity can be judged. One great advantage of using specific gravity as a means of identifying gems is, that the determination can be made without danger of injuring the stone, which is more than can be said of tests of hardness, fusibility, or behavior with acids.

While specific gravity can usually be used for distinguishing between gems, as, for example, between quartz as compared with diamond, it cannot always be used for identifying glass, since by the addition of different ingredients it is possible to make glass of varying specific gravity, and similar to that of the gem which it is sought to imitate.

The specific gravity of a substance is its weight as compared with that of an equal volume of water. When it is stated that the specific gravity of topaz, for example, is 3.55, the figures simply mean that a given volume of topaz is 3.55 times heavier than the same volume of water.

Various means may be taken to determine the specific gravity of a body, the most obvious and simple depending upon the fact that a body heavier than water loses, when weighed in that liquid, a weight equal to that of an equal volume of water. Hence by weighing a body first in air and then in water, and dividing the weight in air by the difference between the weight in air and the weight in water, or in other words, by the loss of weight in water, the quotient will be the specific gravity.

The following example of a determination of the specific gravity of a sapphire will illustrate this :

The weight in grams in air was 12.89
The weight in grams in water was 9.68
Difference 3.21
12.89 = 3.21=4.015, the specific gravity.

A similar quotient will be obtained whether large or small pieces are taken for determination, the specific gravity being totally independent of the actual gravity or weight.

The determination of the specific gravity of gems or minerals becomes then a question simply of manipulations by which the relative weights of the substance in water and air can be obtained in the easiest and most accurate way.

The most common and generally the most convenient way of doing this is by obtaining the weights of the stone in water and air directly by means of a delicate balance. . The stone is first weighed in air and the weight recorded. It is then put into a holder of fine platinum wire, bent into a spiral form, and suspended from the arm of the balance.

The length of the wire is such as to allow the stone to become completely immersed in a vessel of water supported on a stand above the scale pan, but in such a manner as to allow the pan to swing free. In this way the weight of the stone and wire in water can be accurately taken. The stone is then removed, and the wire weighed suspended in the water as before. The weight of this is subtracted from the previous weight, so as to remove the weight of the wire from the calculation, and the remainder is the loss of weight of the stone in water. Dividing the weight of the stone in air by this remainder gives, as stated above, the specific gravity.

Several precautions need to be taken to insure accurate results. In the first place, only distilled water should be used, as ordinary waters have higher density. Again, bubbles of air often adhere to the surface of the stone, especially if it be rough, or if it is pervaded by cracks, which would obviously, if allowed to remain, lessen the weight. These can sometimes be removed by dipping the stone in water several times and blowing the water off, or they can surely be destroyed by boiling for a few minutes the water in which the stone is immersed, and then allowing it to cool before the specific gravity is taken.

Strictly speaking, the specific gravity of a body is its weight compared with that of water at the temperature of 4 Centigrade (39.2' Fahrenheit), which is the point at which the density of water is the greatest. Determinations at other temperatures should, therefore, if absolute accuracy is desired, be corrected to 4 C. In practice, however, the error is so trifling that it may be disregarded in all ordinary determinations, especially if the temperature of the water is no higher than that of the ordinary living room, say 60' F. (15.6 C.).