kimchi

This can get very technical but essentially specific gravity is a number that determines the average density of the item
being tested. it can then be matched up to a table to determine the purity of possible gold and silver content.
fine Gold has a specific gravity of 19.3 and fine silver 10.49.
These numbers are also known as densities measured grams per cubic centimeter.
Many stackers are concerned about fakes.
Having confirmed weights and dimensions another test is to make a specific gravity test.
Sometimes measuring coin thickness relies on averages - centre thickness, rim thickness etc and some wear and tear perhaps so to eliminate doubt make another measurement - the SG test.
One quick way to measure SG is to use precise electronic scales and follow these steps -
1. Weigh your coin in grams e.g. 31.2g
2. Place a paper ( or lightweight ) cup on the scales and fill half full with cold water.
3. Tare i.e. zero the scales
4. Support your coin in a cradle ( cross ) of very fine thread and dip it into the water ensuring it is completely submerged AND not touching the bottom.
5. Read the scales e.g. 2.97g
6. The ratio of 31.2 / 2.97 = 10.5 confirming silver
Metal

Density

Gold

19.3

Silver

10.4 to 10.6
Platinum

21.4

Palladium

12.0

Copper

9.0

9ct

10.9 to 12.7

14ct

12.9 to 14.6

18ct Yellow

15.2 to 15.9

18ct White

14.7 to 16.9

22ct

17.7 to 17.8

Sterling Silver

10.2 to 10.3

950 Platinum

20.1

 
This gets a tad more complicated with say a gold sovereign.
You need to work out ratios knowing that the gold content is 22/24 and the other part 2/24 is either silver or copper.
whilst the sg test works well for silver due to it's relative low value and the
possible cost of ensuring a counterfeit passes this test. it's different for
gold which has a close density replacement in the form of tungsten(sg 19.6).
thus the sg test alone should not confirm gold on larger pieces where
drilling and tungsten filling is known to exist.