The density of a gemstone is another way of differentiating between two similar looking gems, say a garnet and a ruby. While the two may look similar and may even be the same dimensions, each has a unique density, or specific gravity. In case you do not remember this term from high school science, density refers to how much mass something has per unit of volume. Another way to think of density is to think of it as the compactness of an item. Oil, for example, is less dense than vinegar, which is why when a salad dressing separates the oil layer floats on top of the vinegar and spices.
Density and specific gravity are bulk properties that are independent of direction and are typically uniform within a mass of material. While under ideal circumstances these properties are the same for two gems of the same species and variety, this is not always the case in the gem world. In actuality, the density of a mineral varies widely, even within a single crystal, due to the presence of impurities, cracks, and bubbles. Density is a useful parameter in gem identification, so the problems in its determination should be well understood.
Specific gravity is the ratio expressing the weight of a given material compared to that of an equal volume of water at 4⁰C. Thus, a specific gravity of 3 means that, at 4⁰C, one cubic centimeter of the material in question weighs 3 times as much as one cubic centimeter of water.
The density of a compound is a function of several factors, including chemical composition and crystal structure. For example, consider diamond and graphite, both of which are crystalline forms of the element carbon. Diamond has a density of 3.5 because the carbon atoms are tightly packed together in the structure whereas graphite, with a much more loose, open structure, has a density of only 2.2.
The density of minerals within a solid solution series may vary linearly with change in composition. The effect of chemical substitution is seen dramatically in the case of the orthorhombic carbonate minerals aragonite and cerussite. Aragonite is CaCO3 and has a specific gravity of 2.95; cerussite, with the same structure, is composed of PbCO3 and has a specific gravity of 6.55! This clearly shows the role of lead versus calcium in the structure.
Specific gravities are usually measured with heavy liquids. A liquid is prepared, such as a mixture of bromoform and toluene, to have a specific density value. An unknown material dropped into the liquid may sink, float, or remain suspended in one place within the liquid. If the material sinks, it is denser than the liquid, and if it floats it is less dense. If it remains at one level it has the same density as the liquid. Very accurate measurements of specific gravity can be made by changing the density of a column of liquid through temperature variations and suspending density standards in the column.
An alternative method of measurement is the use of so-called torsion balances, such as the Hanneman balance and the Berman balance used by mineralogists. These devices are designed to weigh a sample first in air and then suspended in a liquid, such as water or toluene. The weights in both media can be measured quite accurately and specific gravities can sometimes be reported to two decimal places.
A major problem in all density measurements is the presence of impurities within the crystal being studied. These impurities hardly ever have the same specific gravity as the host material, and their presence results in measurements that are of limited use for identification purposes. Surface tension may also float a mineral grain in both heavy liquids and a torsion balance, resulting in an erroneously low specific gravity measurement. Accurate density measurement involves absolute cleanliness, great care in specimen preparation, accurate temperature control, and replicate measurements.