Purity of Silver is measured in percentage (%). Nines are an informal, yet common method of grading the purity of very fine precious metals.
To get its value, millesimal fineness is used; a system of denoting the purity of platinum, gold and silver alloys by parts per thousand of pure metal by mass in the alloy. It is usually rounded to a three figure number, particularly where used as a hallmark, and the fineness may vary slightly from the traditional versions of purity.
Sterling Silver - Millesimal Fineness of Sterling Silver via kwout
The most obvious way to find out if silver is pure – and the degree to which it is pure – is to check markings on your silver jewelry and silverware.
Markings traditionally measure the percentage of silver present in the metal mixture you own. For example, a metal that is “975″ silver is actually 97.5% silver. If it says “Sterling .925,” likewise, it would be 92.5% silver. These markings should be clear and obvious for your larger silver possessions, and if not, try another purity test before your alarms go off.
There are many different types of silver available so some tips on buying silver can be very useful. Take a look at the various types of silver available at Silverprice.org.