Ok...
There are no secrets in jewelry when it comes to the metals. Seriously, this is a very well studied, analysed, picked apart area.
Really.
More important, there is no such thing as "white" gold. For all the real joailliers, such as those on Place Vendôme, and all serious jewellers, by law in France, to call anything gold, you must have a minimum of 75% (so 18k out of 24k) of the alloy be pure Aureus, ie.e gold. BY LAW. The remaining 25% can be anything but most put silver, with more or less copper. For the "rose" gold effect they'll max up the copper. For "white" gold they'll max up the silver and then have it plated. Yes as most "white gold" is actually quite yellow under the plating; see, otherwise it is not real gold. At 75% gold, you piece can only be yellow. You have to plate it with something to make it white. Usually rhodium. And such pieces have to be replated on the regular.
That's why so many people go straight to platinum. It's less white, more gray, like silver, but you do not get the oxidization of the silver, nor do you have to replate it regularly as the rhodium wears out. I'll just *never* understand why anyone wastes money on "white gold" when it is just plated yellow gold. Get straight up platinum...
Ain't no *white gold* out there. All gold is yellow.
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Now, after that, to get their hallmarks, ALL JEWELRY HOUSES HAVE TO SEND THEIR PIECES TO GET TESTED FOR THE ALLOY BY THE STATE. If your piece of jewelry bears a hallmark, then its contents got tested. AIN'T NO SECRETS IN ALLOYS!
Finally, anyone can buy anybody's pieces and melt them down and analyse them and pick apart the metals composing the alloy. Really. It is not that complicated, people have been doing that for centuries...
It is NOT some secret alchemy.
Please, for your own sake, take some time to read material other that marketing and/or promotional material from jewelry brands. Like actual manufacturing, characteristics, chemical processes etc. about metals and metallurgy. Marketing material is not real knowledge.
And FACTS DO MATTER!
en.wikipedia.org