I spent 16 years in the jewelry industry (retail end) and I know what you are talking about when it comes to plated items. I never even wanted to stock the stuff because it was so much trouble. Here are some tips for jewelry buying.
24K or 24karat is pure gold. when you hear 18K or 14K (the most popular), or 10K this is gold mixed in with a base metal. Copper and nickel are common metals to use. If you have had an allergic reaction to a gold ring it is probably a nickel allergy which is quite common. 8 Karet is what is refered to as Gold filled. Plated is just a tiny sheet of gold plated over a base metal. Similar to paint over wood. Looks good for awhile but wears off. All gold products should be stamped somewhere, Rings are usually stamped on the inside (14k, 10K, etc) Sterling silver is usually stamped as "sterling" or 925 which is the European mark. If there is a "P" after the mark it is plated.
Of course people can get or stamp jewelry on their own and pass it off as real stuff. Most jewlery companies have there onwn tradmark also stamped inside the ring. There are also electronic testers out there which can not only tell if something is gold or not but also what karat gold it is.
And yes, I can tell the difference between a diamond and a cubic zirconia (diamonique) just by looking at it.
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