Testing gold, specific gravity (relative density) method for testing metals and gemstones

Last updated: April 30, 2025

BUY NOW: CLICK ON A PICTURE

Large scale, MD-200S by Mirage £995.00 with specific gravity chamber is for large items.
Small scale with the Quicktest Specific Gravity Kit £169.00 is for small items.
Sovereign Tester weigh the coin (scale included), if the diameter and thickness are correct on the gauge, it's 22ct gold.
American Specific Gravity Kit £95.00 (not illustrated above) scale not included, you must supply your own.

SUMMARY
SPECIFIC GRAVITY (relative density) - the theory
SPECIFIC GRAVITY -THE PRACTICE
TWO SPECIFIC GRAVITY KITS COMPARED
CONCLUSION

SUMMARY

Testing by specific gravity (SG) does not require filing the item and the only chemical involved is water (with the sovereign gauge you don't even need water) You can test precious metals (in fact, any metal!), loose gemstones and small mineral samples.

The remainder of this article describes how it works, using weighing scales and water.

Advantages

This is a 100% non-destructive test. Other methods (including electronic tests) require metals to be filed first (to remove any surface plating); also, the acid test can leave a stain. The stain can be polished off easily, but that's not the point - if you have a rare coin you cannot file it or stain it.
Specific Gravity is suitable for testing all metals, not just precious metals, whether in the form of jewellery, coins, small bars or just bits of unknown scrap
you can test gemstones
you can test mineral samples.

Disadvantages

it takes a few minutes to set up and each test involves great care and patience, it is not something staff can do whilst under pressure with a queue of customers.
the item must be homogenous, i.e. jewellery must be made of one metal only, no 'extras' such as gemstones; likewise, mineral samples must be made of one material only (crystals are good).
you can only test small items, up to (approximately) 25mm diameter X 15mm. Larger items will not fit. You cannot test very small gemstones, the test isn't sensitive enough.

SPECIFIC GRAVITY (relative density) - the theory

Density is the amount of 'stuff' in a given space. Take a wedding ring made of aluminium and an identical-looking item (same size) made of platinum. The platinum wedding ring will feel over eight times as heavy as the aluminium wedding ring. You will lift the aluminium item and say, "That's light!" You will lift the platinum item and say, "That's heavy!".

Of course, that's not what you mean. You mean, "That is much lighter / heavier than I expected, for the size." It is this 'being heavy or light, for the size' that is density, how much 'stuff' is squished into the space.

For the sake of the physics (which I shan't explain here) a cube of water measuring 1cm X 1cm X 1cm weighs 1g. Specific gravity is: how much a 1cm X 1cm X 1cm cube of unknown material* weighs compared with 1cm X 1cm X 1cm of water.

* This can be any 'material', e.g. metals, gemstones.

So, for instance, a result of SG 6.5 would mean your metal is 6.5X heavier than the same volume of water; for instance, a result of SG 1.5 would mean it was 50% heavier than the same volume of water. As you may notice, you don't actually have to cut the items into 1cm X 1cm X 1cm squares!

But please don't worry if you don't understand any of that, you only need to know how to take the measurement.

SPECIFIC GRAVITY -THE PRACTICE

Weight the item on the scale (weighing machine) as normal. This is the "weight in air".

Then the item is placed in the basket which is lowered into the little beaker of water...and weighed. This is the "weight in water".

Find a calculator
a) Take the smaller number away from the bigger number
b) Divide the answer into bigger number
c) Look up the chart (there's one chart for metals and one for gemstones).

CHART FOR METALS

CHART FOR GEMSTONES

As you notice from the chart, pure metals have single values (though even these can vary) whilst 'impure' metals (alloys containing mixtures of metals) have a 'range' of values, and there are overlaps which can make it difficult to distinguish between 14ct and 18ct, or 18ct and 22ct, or copper and 9ct.

It is, however, invaluable if you already have clues:

You think you have a platinum wedding ring, you get a reading of 16.5, it might be white gold, it can't be platinum.
You think you have a Victorian 15ct brooch, you get a reading of 10, it might be 9ct, it might be copper, but it can't be 15ct.
You think you have a gold Sovereign, you know they are made of 22ct gold, you get a reading of 12, it can't be 22ct, it is a forgery.

It is especially useful for coins, because you know exactly what the genuine coin is made of, and you can also look up the exact weight and carat in The Gold & Silver Buyer's Handbook).

TWO SPECIFIC GRAVITY KITS COMPARED

American Specific Gravity Kit £95.00

CONS:

Fiddly to assemble, fragile, made mostly from of wire
Good if you already have a suitable scale
Old stock, we only have one (as of May 2025)

PROS:

Very cheap (as testers go)

Quicktest Specific Gravity Kit £169.00

CONS:

More expensive than the American model

PROS

Sturdily-made and easier to assemble
Scale included
Our (Quicktest) own brand, made in England

A SOVEREIGN (22ct)

I used the large specific gravity (SG) attachment and items weighing between 5g and 8g. I wanted to see how good the small self-assembly models were. If you were to use the large table balance with the specific gravity chamber (£995.00) you would get more accurate and more consistent results.

The following tests were carried out on a sovereign (22ct). It weighed 7.987g. We know from the text books that it should weigh 7.988g plus-or-minus 0.1g so if I were testing this to see if it were a genuine Sovereign, finding that the weight is correct is a good start. Officially (there is always variation) 22ct should be between 17.7 and 17.8.

I took five readings and got 18.22, 17.59, 17.55, 17.630 and 17.49. Average: 17.76. Although a tiny bit low, this is an excellent reading.

A BAR OF PURE (24ct / .999) GOLD

WEIGHT, 5g. Specific gravity should be 19.3.

I took five readings and got: 19.40, 19.90, 19.55, 19.85 and 18.85. Average 19.51. Although a little high, this is fine, we would expect readings to be correct to within 0.5 so an error of only 0.2 is good.

CONCLUSION

Specific gravity is the recommended method to test rare and delicate antiques, collectors' coins, and anything which must not be marked in any way. It's also good for gemstones over about 1 carat (0.2g) in weight and small mineral samples. You do need patience, the method is slow, you must take a few readings and take the average.