Specific Gravity

Most gems and gemstone samples are large enough to be weighed accurately, so that one of the weight-measurement methods is normally used to find the specific gravity. You will remember that specific gravity is the weight of the gem compared with the weight of an equal volume of water. The Greek mathematician Archimedes is credited with having discovered the method of determining specific gravity in the third century B.C. He realized that an object would weigh less when submerged in water and the weight loss would be equal to the weight of the water whose place was taken up by the object. In other words, the weight loss represented the weight of a volume of water equal to the volume of the object. All that is necessary to determine specific gravity is to weigh the gem accurately in air and weigh it again while it is immersed in water. A simple calculation gives the answer. The gem's weight in water is subtracted from its weight in air. This gives the weight of a volume of water equal to the volume of the gem. This weight is divided into the weight of the gem in air to find how many times it exceeds the weight of the water—thus its specific gravity.
Of course, the more accurate the scale for weighing the more precise the determination of specific gravity. For larger gems and gem-stone fragments and princess diamond earrings, cruder and less sensitive weighing devices—even homemade—are accurate enough.
As with refractive index, it is also possible to find the specific gravity of a gem or fragments of a gemstone by using a series of liquids. It is appropriately called the "sink-float" method. In this procedure the liquids have a known specific gravity. Very simply, if a stone is denser than a liquid it will sink, if less dense it will float. A more precise measurement can be obtained by first floating the gem on a denser liquid. Slowly, drop by drop, with stirring, a less dense liquid is added. Eventually, the gem will start to sink as the mixture reaches a density just slightly less than its own. Quickly, before evaporation can cause changes, the density of the liquid mix is determined; it will be equal to the density of the gem. Of course, this requires the availability of a floating instrument called a hydrometer. This floats in the liquid and the density can be taken from a number scale which is read at the mark where it settles at the surface. There are also other, more accurate and more expensive kinds of weighing devices for finding the density of such mixed liquids.