Heavy liquids are substances in liquid form that have a known Specific gravity (SG) and are used as a means of quick separation of stones with similar appearance. In general, these liquids are highly toxic and precautions should be taken when used.
- 1 Basic
- 1.1 Safety warning
- 1.2 Use of heavy liquids
- 1.3 Types of heavy liquids
- 1.4 Calibration of heavy liquids
- 2 Sources
Before using heavy liquids, it must be stressed that one should avoid all contact with the skin and that they are non-potable. Always use tweezers to insert the gemstones and, if liquid comes in contact with the skin, you should wash thoroughly. When in contact with eyes, you should rinse your eyes with streaming water and contact your medical doctor immediately.
Always use heavy liquids in a well-ventilated environment. They should be kept away from children and pets.
Use of heavy liquids
The use of heavy liquids is fairly straightforward. When a stone is immersed into the liquid, it will either float, suspend, or sink.
When the SG of the liquid is known, take the gemstone in your tweezers and gently lower the stone in the liquid. Observe the stone while looking through the side of the bottle and take the stone out again with your tweezers.
Carefully rinse the stone and tweezers under streaming water (you might want to take precautions to not let the stone go down the drain with the water), or you could rinse it in a large beaker filled with clean water.
After your observations, you should be able to tell if the stone’s SG is higher, lower or equal to the heavy liquid used.
- When the stone floats in the liquid, it will have an SG lower than the liquid.
- When the stone is suspended in the liquid, SG will be equal to the liquid.
- When the stone sinks, it will have a higher SG than the liquid.
One can also observe to which degree a stone will sink or float after suspending it in the liquid.
For instance, when the stone sinks rapidly it will have a far greater SG than the liquid.
Types of heavy liquids
In gemology, generally 4 heavy liquids are used.
- Methylene Iodide - SG = 3.33
- Methylene Iodide diluted with Toluol - SG = 3.05
- Bromoform - SG = 2.85
- Bromoform diluted with Toluol - SG = 2.65
In addition to these four there is Clerici Solution with a SG of 4.2, used in laboratories. This solution can be diluted with water to lower its SG, such as SG = 4 or SG = 3.52 for instance. This solution is extremely toxic and should only be used in a well equipped laboratory with additional safety precautions such as an acid chamber.
Dr. W. Wm. Hanneman suggests the use of Sodium Polytungstate as a non-toxic alternative for most heavy liquids.
Aside from these, there is also a water/salt solution for identifying Amber.
Methylene Iodide (CH2I2) is also named di-idiomethane and has a SG of about 3.33. Yellow Topaz (3.5) will sink in this liquid while Citrine (2.65) will float.
Methylene Iodide diluted with Toluol
Methylene Iodide diluted with Toluol has an SG of 3.05. This liquid can easily separate Tourmaline (3.05) from Topaz (3.5) because Tourmaline will suspend in this liquid while Topaz will sink.
Undiluted Bromoform has a SG of 2.85 and can be calibrated to a lower SG with the addition of Toluol.
Bromoform diluted with Toluol
Diluted Bromoform with an SG of 2.65 is the indicator for Quartz.
This highly toxic Thallium-based liquid has a SG of 4.2 and can be diluted with water to calibrate it for Corundum (4) or Diamond (3.52).
Sodium Polytungstate is a commercial, non-toxic and odor free alternative for most heavy liquids. It is sold as a white powder to which water is added to form a solution. The upper value of the saturation is 3.10. By adding water, it can go down to whatever SG is preferred.
Lithium Metatungstate (LMT)
Another common heavy liquid is Lithium Metatungstate (LMT). It is sold at 2.95 specific gravity and can be diluted with distilled water down to 1.0 Specific gravity. LMT is safe to use and it is the most affordable inorganic heavy liquid on the market. Visit LMTLiquid.com for more information.
When 7 grams of ordinary salt (Halite) is saturated in 50cc of water, a solution with density of 1.12 is created. This permits Amber (SG = 1.08) to float in it while some simulants (most of which - but not all - are plastics) sink.
Calibration of heavy liquids
Since evaporation, temperatures and light conditions may effect the SG of the liquids, it is recommended to have calibration stones at hand so one can periodically adjust the solutions. Changes in temperatures especially can highly influence the calibration.
If the calibration stone suspends in the liquid it will be up-to-date, otherwise one should adjust the solution.
Generally, one calibrates a heavy liquid by dropping a calibration stone slowly into a liquid and diluting it drop by drop until the stone of known SG suspends in it.
- Gemmology 3rd edition (2005) - Peter Read
- Guide to Affordable Gemology (2001) - Dr. W. Wm. Hanneman, Ph.D.
- Journal of Gemmology, Vol.22, No. 6 (April 1991) - Gem-A
- Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (6th ed.)
- Gem-A Foundation notes - Gem-A