Specific gravity (relative density) method for testing gemstones (and metals)

 

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SUMMARY
SPECIFIC GRAVITY (relative density) - the theory
SPECIFIC GRAVITY -THE PRACTICE
EXAMPLES, test results (gemstones and metals)
NOT ACCURATE ENOUGH?
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, plus examples of test-results when using the QUICKTEST specific gravity tester.  

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; you must work slowly and carefully, 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 (clear crystals are good). 
  
• maximum size of sample is about 25mm diameter X 15mm; minimum weight of sample is about 0.2g / 1 carat (you can test smaller gemstones but the result might not be accurate).  
  

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

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

Then place the item in the basket, lower it into the water, weigh it. 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). 

SPECIFIC GRAVITY CHART FOR METALS

SPECIFIC GRAVITY CHART FOR GEMSTONES

As you notice from the metals chart, pure metals have single values 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 9ct and copper.

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 carat and weight should be. 

EXAMPLES, test results (gemstones and metals)

Here are some examples of tests carried out using the QUICKTEST specific gravity tester. 

GEMSTONE: MOISSANITE, should be 3.22 (weight 0.352g / 17.5 carat) 
Average reading was 3.28. This is a bit high but close enough to distinguish it from diamond (which is 3.52).

GEMSTONE, DIAMOND: should be 3.52 (weight 0.122g / 0.6 carat).
Average reading was 3.59, however the individual readings varied from 2.97 to 4.2 which shows that (as we say) this tester is not accurate for samples less than 0.2g / 1 carat. 

GEMSTONE: RUBY: should be 3.97 to 4.05 (weight 5.442g / 27.2 carat)
Average reading was 3.95, a bit low but close enough. 

GOLD: A SOVEREIGN (22ct): should be 17.7 to 17.8 (weight 7.987g)
Average: 17.76. Although a tiny bit low, this reading is close enough. 

GOLD: BAR OF PURE (24ct / .999): should be 19.3
Average 19.51. Although a little high, this reading is close enough. .

NOT ACCURATE ENOUGH?

Then go for the 'Densimeter' MD-200S by Mirage with specific gravity chamber, special offer price £995.00 (ex-display) - these are usually £1,500.00 to £2,000. 

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.