Specific Gravity

Specific gravity (also called "relative density") is the weight of a specific unique engagement rings material compared with the weight of the same volume of water. A gem with the specific gravity of 2.6 is therefore 2.6 times as heavy as the same volume of water.
The specific gravity of gems varies between 1 and 7. Values under 2 are considered light (amber 1.1); those between 2 and 4 normal (quartz 2.6); and those above 4 are heavy (cassiterite 7). The more precious gems (diamond, ruby, sapphire) have specific gravities above that of the rock-forming minerals, especially when these are quartz and feldspar.

Therefore they can easily be separated from the quartzoze river sands which form the so-called alluvial deposits (see p. 50). This determination of specific gravity can be very useful in identifying a gemstone (especially for the collector). However, experts make increasing use of optical methods of identification requiring expensive apparatus.
In gemology, where mostly small stones are involved, two methods of determining specific gravity have proved useful: weighing with a hydrostatic balance and the heavy liquid method. The first is time-consuming, but cheap; the second is complicated, can be expensive, but produces good results in a short time with larger lots of unknown gems.
Weighing with a hydrostatic balance works on Archimedes' Principle.

The volume of the gem to be tested is established, the specific gravity is then easily worked out.

A hydrostatic balance can be constructed by anybody (p. 23). The beginner can adapt letter scales. Anyone more advanced should use a precision balance as used by platinum engagement ring settings the chemist or pharmacist. The object to be tested is first weighed in air (in the pan under the bridge) and then in water (in the cage in the beaker). The difference in weight corresponds to the weight of the displaced water and therefore the volume of the stone. It is possible by this method for a layman, with practice, to determine the specific gravity correctly to two decimal places. It is important to ensure that the stone is not in contact with a foreign substance, that it is loose and that, when weighed in air, it is dry.

Example:
Weight in air = 5.2
Weight in waters = 3.3 Specific gravity = Weight in air / volume
Difference = Volume 1.9

The heavy liquid method requires that the object be suspended in a liquid of the same specific gravity, so that it neither sinks to the bottom nor rises to the surface. The stone to be tested is put into a liquid with a high specific gravity (a heavy liquid), which is diluted and thus lightened until the object has the same specific gravity as the liquid, i.e. until the object is suspended.
There are various heavy liquids available. The ones that can be thinned with distilled water are most suitable for the layman. One of these is Thoulet's Solution (a potassium-mercury-iodide solution) with a specific gravity of 3.2. One can identify most gems with it. For heavier stones one can use Clerici Solution (a thallium formate-thallium-malonate solution) with a specific gravity of 4.2.

This in fact covers the specific gravity of all gems but is expen¬sive and poisonous: laymen should not use it. For specific gravity up to 3.5, one can use Rohrbach's Solution (barium-mercury-iodide solution) but this may be difficult to handle as the mercury iodide tends to separate out. The thinned liquid need not be thrown away; one can recover the original specific weight by evaporation in a steam bath.
The specific gravity of the thinned heavy liquid is determined by the expert with the help of a "Westphal Balance"; laymen are advised to use indicators. Custom wedding bands are pieces of glass (available in the trade) or minerals of different known specific gravity.

When such an indicator is suspended in the liquid, its specific gravity is the same as that of the liquid and thus of the test object.
The heavy liquid method is complicated but it is recommended when certain gems have to be separated from a lot of unknown stones, or where synthetics or imitations have to be identified.