Specific Gravity - A measure of density
Two material objects of the same size, such as a wood block and a gold block, may have the same proportions or measurements but will not have the same density, or therefore the same weight.
Like-sized pieces of one and the same material always have the same weight.
It has long been a practice to compare the densities of solid 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.
Density is mass per unit of volume. Or, the ratio of the amount of matter in an object to its volume.
In the late 1800’s the understanding of scientic process was much easier to grasp because the method of writing of these was much simpler, less complicated by terms and language usage of the profession. Offered here is a summary of what various late 19th and early 20th century writers told of the why and how to learn of the specific gravity of minerals.
Why determine specific gravity and use the number with gems and precious stone?
Many stones, like the ruby and the spinel, the blue tourmaline and the sapphire, etc., look alike, but there is a sensible difference in their respective weights, the weight number that a specific-gravity test will readily establish.
One of the most important tests which can be applied to a polished stone is that of specific gravity.
The weight of an object which is free to seek the centre of gravitation is called absolute weight, while the weight of an object compared with that of another containing the same volume of matter is called the specific weight.
If a stone weighing 16 carats is placed in a vessel filled to the brim with distilled water and the stone displaces 6 carats of water, the specific gravity of the stone would be determined as 16 ➗ 6, or 2.66, which is the specific gravity of quartz.
In other words, the stone would weigh 16 carats in the air and only 10 carats in the distilled water, showing a loss of 6 carats, which is the weight of the volume of water equal in bulk to the stone ; - or absolute weight, 16 carats; specific weight, 10 carats; loss, 6 carats; 16 ➗ 6 = 2.66 -the specific gravity number.
There were several methods of ascertaining the specific gravity of a stone, but most practical of the time was by weighing the stone in air and then in distilled water or alcohol, and thus learning the weight of an equal bulk of water.
It was very seldom, if ever, the case that any two precious stones of the same specific gravity were found. A few stones have nearly the same specific gravities, and in such cases it is well to apply other tests also. In fact one would always make sure of a stone by seeing that two or three different tests point to the same species.
An example from another book of lessons explains this way;
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. We now need only divide the weight in air by the loss of weight in water, and we shall have the specific gravity of the stone.
The specific gravity method is of value in distinguishing between the various colorless stones, as, for example, quartz crystal, true white topaz, white sapphire, white or colorless beryl, etc. These stones are all doubly refractive, have no color, 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.
But, 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.
SIDE NOTES
| GLOSSARY SOURCES |
INFO sourced from publications of the late 19th century and early 20th century. Handbook of Precious Stones 1890; A Book of Precious Stones 1909; A Text-Book of Precious Stones 1918
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| CARAT. (Karat.) | A unit of weight applied to precious stones varying in different trade centres. The word carat is supposed to be derived from "Kuara," the bean-like fruit of an African tree reputed to have been used as a standard of weight for precious stones. |
| K a r a t | is used to indicate degrees of quality in gold. |
| TESTING |
Distilled water or fluids must be of specific temperature for accurate determination. Care must be made not to transmit warmth from hands to vessels of fluids. |
| Sources | A Handbook of Precious Stones. 1890 A Textbook of Precious Stones 1918 Britannica online. 2025 |
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