In regards to soaking turquoise that is set in jewelry -Soaking can cause some pieces with sawdust/wood and other material placed behind the stones used as fillers in the settings to expand which can displace and crack stones. Also treated city waters and some well waters will discolor metals used in jewelry.
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that is so true. Thanks for pointing out that. we never soak finished pieces. also, we only use spring water or distill water for that purpose.
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So, when you say “ID a piece”, you’re talking about an individual stone, not a piece of jewelry. This makes more sense. I couldn’t imagine asking a dealer to submerge an expensive piece of jewelry in water for 20 minutes to see if the stone changes color. 
Back to my original question, it sounds like it would be difficult to tell if a stone is stabilized or not when purchasing a piece of jewelry.
Sorry for the confusion. My English isn’t great and I work at the manufacture stage instead of retailing stage of the jewelry business so a “piece” is always the individual stone instead of the final works.
Back to you question, if the stone has been already set, then you really have to use analytical method to be sure. Although if the metal part is in gold, I can’t think of any harm a water submerge can caused.
Also keep in mind that an old piece of jewelry could fail the water submerge test even the stone is perfectly natural since human skin is full of grease and it will get into the stone and slowly change its color (it takes about 2-3 months if you allow that piece directly contact against your skin) and this process will seal the stone just like those polymers. In this case, even simple analytical method cannot distinguish between stabilized and human skin grease infused piece. You need even fancier tests to do so.
So yeah, it is very complicated and that’s why I always keep a picture of the rough before cut and before polish my stone into final product. In this way, you can almost ensure the customer this piece hasn’t been infused with polymer. If someone really like the color of the freshly cut stone, I’ll apply a thin layer of polymer coating and leave a laser mark so I can tell this piece has been “stabilized” later. The top coating won’t get into the stone too deep and if someone really want to check, they can simply sand down about 5um and they will reach the raw material again.
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