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Without using your eyes, can you tell two gemstones apart ? Maybe. The density of different stones can lead you to their true nature. Today’s episode discusses what specific gravity and how it is measured, both now and in ancient history.
The Gemological Institute of America (GIA) offers a set with densities of 2.57, 2.62, 2.67, 3.05, and 3.32. Other manufacturers offer sets with different ranges.
Before Using Heavy Liquids, Know the Risks
Most heavy liquids are solvents, in the same family as thinners, and toxic. Don’t inhale their vapors or get any on your skin or clothing. Heavy liquids are also flammable, so keep them away from heat sources at all times. Follow all the manufacturer’s safety instructions for whatever set you choose.
Avoiding Lapidary Health Hazards: Proper Ventilation and Protective Gear
The major health hazards lapidaries face are dust (inhalation, ingestion, and contact with the eyes) and dermatitis due to improper skin protection.
Proper ventilation is necessary for any lapidary or jeweler’s workshop. Between the dust and odorous chemicals, there’s good reason to invest in a ventilation system. You may want to hire an HVAC expert to assess your particular needs.
Always ensure that your ventilation intake brings in clean air, not affected by car exhaust, chimney smoke, or another pollutant source. Take care of the system and replace the filters frequently to avoid clogging.
Dust and mists produced from lapidary work should be sucked away as close to the source as possible. The closer, the better to avoid breathing in particles. An even more effective precaution, a glovebox will enclose entirely any dust and mists produced. It’s better to have an enclosed system to contain the dust and mist than to try and fully enclose yourself.
Even with proper ventilation, wear a dust mask so that the particles that inevitably escape don’t settle in your mouth or lungs. Tight-fitting safety glasses or goggles will protect your eyes from dust irritation as well…
Why Use Heavy Liquids?
The theory behind using heavy liquids is simple. If you put a gem in a liquid with the same density, it’ll stay in place. Should the gem have a greater density, it’ll sink. So it follows that the gem will float if it has a lower density.
How fast the gem sinks or floats will give you some indication of how much its specific gravity varies from the liquids. If the difference is just a couple hundredths of a point, it will drift slowly up or down.
Specific Gravity
Specific gravity (SG) or density refers to how much something weighs in relation to its size. For example, a cubic centimeter of steel is much heavier than a cubic centimeter of styrofoam. SG is expressed as the relationship to an equivalent volume of water. For example, a gem with a specific gravity of 3 weighs three times as much as the same amount of water.
In gemology, the terms specific gravity and density are interchangeable.
Specific Gravity Values of Popular Gemstones
| Gem | SG |
| Garnet | 3.40-4.25 |
| Corundum | 3.97-4.03 |
| Topaz | 3.53-3.56 |
| Diamond | 3.51-3.53 |
| Peridot | 3.27-3.48 |
| Tourmaline | 2.84-3.10 |
| Beryl | 2.66-2.80 |
| Quartz | 2.63-2.68 |
| Opal | 1.99-2.25 |
| Amber | 1.05-1.096 |
As you can see from the chart, a few gems share the same SG range. This makes SG one of the most useful gemstone physical properties you can have for identification. However, measuring specific gravity is difficult and time-consuming. Thus, you normally only conduct this test when necessary.
Using Specific Gravity to Estimate Gem Weight
Specific gravity can be used to estimate the weight of a gemstone. By knowing the specific gravity of a gem and its dimensions, you can calculate its weight using the formula:
Weight = Specific Gravity x (Length x Width x Height) x 0.008
For example, if a gemstone has a specific gravity of 3 and its dimensions are 10mm x 8mm x 6mm, the estimated weight would be:
Weight = 3 x (10 x 8 x 6) x 0.008 = 1.44 carats
It’s important to note that this is an estimation and the actual weight may slightly vary due to variations in gemstone density and shape.