Testing gemstones using the Specific Gravity (relative density) method
There is also a very good book on the subject
To buy a specific gravity testing kit, click here
To buy chemical testing sets click here
The components: frame with platform that hangs in the water, frame with beaker of water, bottom support enables you to weigh the item on the weighing platform ('weight in air') without dismantling the kit. THE WEIGHING MACHINE IS NOT INCLUDED.
The item is weighed on the weighing platform
Then the item is weighed on the platform that is suspended in the water
Make the calculation listed below...
...and look up the chart.
(there's a chart for metals and a chart for gemstones)
Testing loose gemstones using the specific gravity method is a standard method in gemmology. This includes diamonds. The stones do have to be loose, you cannot test stones that are mounted in jewellery.
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 ring (same size) made of platinum. The platinum 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 / much 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) we 'expect' that a cube of water measuring 1cm X 1cm X 1cm will weigh 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. [When I say, "unknown material" I mean 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; or 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 practise
In practice you really don't need to know about heavier-than or lighter-than or the weight of water. In practice all you need to know is how to take the measurement:
Weigh the item. This is the weight in air.
Now weigh the item while it is suspended in water (this is what the specific gravity kit does). This is the weight in water.
Take the smaller number away from the bigger number. To aid your memory, draw a circle around the answer.
You will now, probably, need a calculator. Calculate: item weight (weight in air) divided by the number you circled.
The answer is the specific gravity (SG).
Now look up the standard chart for gemstones.
SPECIFIC GRAVITY KIT
Assembling it: surprisingly easy, you can see by the picture. The kit is delicate (especially the 'frame' that holds the equipment and the 'hanging wire' that holds the weighing platform in the water) so you do need to handle everything with care, if you're the type of person who is always knocking glasses of water over, you really won't have much a chance when it comes to using a specific gravity kit. But it is quite easy to assemble and using it is not difficult providing you work slowly and carefully. Carrying out a test, and taking a second or third test, just to be certain, and checking your calculations - all that can take four or five minutes, it is not 'instant'.
Instructions for use:
- weigh the item. This is the weight in air
- weigh the item while it is suspended in water. This is the weight in water
- take the smaller number away from the bigger number. To aid your memory, draw a circle around the answer
- find a calculator
- calculate item weight (weight in air) divided by the number you circled
- all figures for 'specific gravity' are approximate, actual densities vary because nature does not make gemstones to standard formulae, there are always slight variations.
- figures will vary with temperature
Reasons for errors in the readings:
- sample too small, I struggle to get accurate results on items weighing less than 0.5g on a cheap pocket balance. Though it depends on the accuracy of the weighing machine not the 'specific gravity' attachment.
- edge of plastic holder touching wire of frame
- the platform (in the water) touching edge of beaker
- scale does not start at zero
- slight spillages = not exactly 50ml in beaker (small model) or 100ml in beaker (large model).
- there is only physically space for small items (see "Two models, two sizes" below) - ample room for gemstones but no room for large mineral specimens.
- you must test un-mounted stones, it won't work on jewellery containing gemstones
- as regards cost, the specific gravity kit does not include the weighing machine, you must use one that weighs down to 0.001g.
- test each item twice, just to make sure you've weighed and calculated correctly
- if the item weighs less than 1g carry out two or three tests
- if the item weighs less than 0.5g carry out three or four tests and if not consistent, take an average
- check level of water after every test, the beaker must be filled to exactly 50mls (small model) or 100mls (large model).
Ideally (but more important for small gemstones bigger mineral samples):
- use distilled water
- the water should be at room temperature, ideally 20 degrees C (68 degrees F)
- make sure there are no bubbles around the metal by very gently stroking the metal with a fine brush
TWO MODELS, TWO SIZES
The two models are identical in construction, but different sizes. The small size is for testing gemstones and small gold items (up to the size of a couple of one pound coins); the larger model is or larger items (up to the size of a stack of five two-pound coins).