A simple way to determine if you jewelry is silver or sterling silver is to purchase a test kit. These kits are usually available from most jewelry supply companies. Some kits are designed for testing both silver and gold giving you even more of a reason to purchase them. Most kits contain solutions which have nitric acid in them so you need to be careful when handling the kit so that you do not accidentally get the solution on your hands. It is also very important to perform tests in a very well ventilated area (I suggest keeping your windows open). Usually there is a glass rod (which does not react to the acid) you can use for dropping the solution onto your jewelry.
If the kit does not come with directions, make sure that you ask for thorough directions! Test on a cleaned area of the jewelry. Some kits advise you to use a file to brighten the area. This will damage your jewelry and is not recommended!
After the solution has been applied to the jewelry it will change colors if there is any silver present. In kits that have a yellow colored solution (colors may vary between manufacturers of different kits), the solution will change from yellow to various shades of red when it comes into contact with silver. A deep burgundy color indicates "sterling silver" (92.5% silver). A cloudy pale reddish color means there is silver but less than sterling's 92.5% standard. If the solution does not change color it is a safe bet that your jewelry is not silver.
Always use the testing solution on an inconspicuous area of the jewelry because sometimes the solution may discolor the jewelry and leave a "testing mark". As soon as you see a color change wipe the solution off of the piece with a soft cloth to prevent a testing mark. Then, use a cloth to re-polish the tested area back to its original finish. The less time that you leave the solution on your jewelry, the less of a chance that a testing mark will appear.
Now get to testing your jewelry!
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