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If you can't find any identifying stamps and you're unsure about the origin of the jewelry, use a testing kit to find out what the jewelry is made of. Purchase an acid testing kit from an online retailer or jewelry supply store. This kit will include a sanding stone and several bottled acids.  Buy a kit that can test for both silver and platinum. The bottle labels will indicate which metal they are used to test for. If the kit doesn't include gloves, buy some separately. If you get any acid on your hands you will burn your skin. Lay out the black slate stone on an even surface. Gently rub the jewelry against the stone in a back-and-forth motion to create a line. Draw 2 or 3 lines on the stone or one for each testing acid you'll use. For example, if you're testing for platinum, silver, and gold, you'll draw 3 lines.  Choose an inconspicuous part of the jewelry to rub against the stone. The stone will scratch and damage a small section of the jewelry. Lay a towel down underneath the stone to protect your work surface from scratches. Select an acid test from your kit and carefully drop a small amount of acid onto one of the drawn lines. Make sure that you don't mix the different acids together or you'll affect your results.  Most tests have acids specifically for silver. However, you can also use an 18-karat gold testing acid to identify pure or sterling silver.  Always wear gloves while handling acid. These reactions can take anywhere from a second to a minute. If the line completely dissolves, the test fails. For example, if you drop the Platinum acid test on a line and the line dissolves, the jewelry isn't platinum. However, if your line doesn't dissolve, the metal is pure.  If you use the 18-karat gold acid test to test on silver, the line will turn a milky white color. This will indicate that your jewelry is pure or sterling silver. If you doubt your results, test the jewelry again to be sure.

Summary:
Use an acid testing kit on hard-to-verify jewelry. Rub the jewelry against the stone. Drop the acids on the different metal lines. Observe the reaction of the acid.