Heavy Liquids

For the purpose (s) of identifying gems it is not usually so important to obtain an exact SG determination because most gems have a plus or minus range of SG. An approximate value is just useful as an exact value. For this reason, heavy liquids offer the gemologist what is considered by most to be the best overall method of SG determination available.

When a gem is put into a liquid of lower density than its own, it will sink. If the stone’s density is less than that of the liquid, it will float. Or if the stone’s density is exactly equal to that of the liquid, it will suspended, neither sinking nor floating. So it can be seen that by using a range of liquids with known densities it is possible to estimate the specific gravity of a gem. By noticing the speed with which a gem sinks or floats in a liquid, an accurate estimate can be made regarding the difference between the specific gravity of the liquid and the stone.

Types Of Heavy Liquids
The heavy liquids today consist of methylene iodide (pure SG=3.32) diluted with varying amounts of benzyl benzoate (pure SG=1.12) to obtain values below 3.32. The main advantage of using benzyl benzoate to dilute the methylene iodide instead of xylene (toluene), which was used in the past, is that the evaporation rates for methylene iodide and benzyl benzoate are nearly equal and so the density of the mixture is fairly constant and does not need to be adjusted as often as mixtures using xylene.

Methylene iodide: 3.32
Bromoform: 2.89
Benzyl benzoate: 1.12
Xylene (toluene): 0.87
Clerici’s solution: 4.15

Clerici’s solution is a concentrated solution in water of thallium formate and thallium malonate. It is expensive, poisonous and rather dangerous to handle compared with other liquids, but with a density of about 4.15 it is easy to forget about these shortcomings. Many find it useful to dilute Clerici’s solution (with water) to about 4.00 which is the SG of corundum.

Of all the heavy liquids, methylene iodide is undoubtedly the most useful. With a pure SG of 3.32 and of equal importance, an R.I (refractive index) of 1.74, it can serve two purposes: testing SG and immersion. Upon exposure to light, methylene iodide has a tendency to turn brown or black, but if a small piece of copper is kept in the bottle this darkening process can be reversed.