How to Measure Specific Gravity of a Gemstone

When we think of gravity, rarely do we attribute it to gemstones. However, the term specific gravity refers only to gemstones, and knowing more about such luxurious and potentially expensive products never hurts. Specific gravity is a means of measuring the density of a gemstone. This measurement is achieved by judging the density of the gemstone relative to the density of water. The specific gravity is expressed by a number that denotes how heavy the gemstone is in comparison to water of equal volume.
For example, a gemstone may be two times heavier than an equal volume of water. In that case, the specific gravity of the gemstone is simply two. The majority of gemstones exhibit a specific gravity that is between two and four times the equal volume of water. But there are of course exceptions. Zircon, one of the densest gemstones in the world, exhibits a specific gravity that is just shy of five.

Specific Gravity In Gemology

Specific gravity is useful for both the gemologist and the gem buyer. For the gemologist, specific gravity can help them identify a gemstone if it isn’t obvious already. As for the gem buyer, the specific gravity can help you understand the difference in size between gemstones, even if those gemstones are the same carat. Stones with higher specific gravity will be smaller than stones that have higher specific gravity, even if both gemstones have a carat of one. This is why sapphires are smaller than diamonds of the same carat.

Specific Gravity of a Gemstone

Specific gravity, Mass and Density

However, it is important to avoid confusing specific gravity with density or mass, neither of which are the same thing. Density differs from specific gravity in the fact that it is mass of an object divided by its volume. As you can plainly see, this measurement is not acquired by the same method specific gravity is, and has nothing to do with a ratio compared to water.
As for mass, it is the amount of material that a specific object is comprised of. In layman’s terms, how much diamond is in my diamond? The mass of a solid object does not change (unless part of it is destroyed or removed), and in that sense it is different from the idea of weight, which is subject it change under various circumstances. ‘Weight’ denotes only how much force gravity is exerting on an object at any one time, meaning that weight is not a constant figure. You weigh more on earth than you do on the moon, and the amount of gravity exerted on you in water is different than on land, making weight an inaccurate but common factor used when judging physical properties of any object, including gemstones.

How To Measure Specific Density

As for how you actually measure the specific gravity of a gemstone, you need a hydrostatic balance. The stone in question is first weighed in air and then weighed again while immersed in water. Once those numbers are received you simply divide the weight of the stone in the air with the difference bweteen the weight in the air and water.
Specific Gravity = (Weight in Air) / (Weight in Air – Water in Water)

specific gravity of a gemstone


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