THere are several websites that detail the composition of common jewelry metals - it's been a few years since I looked thru them but I remember that there is surprisingly wide variety in compositions. Sterling Silver, 925, must contain 92.5% silver, but the other 7.5% does vary. The 'brightness' you describe can be rhodium plating, which John Hardy pieces feature. This is the same metal which coats almost all 'white gold' and gives that white, bright shine. It wears off over time - years - but a competent jeweler can reapply it for a small expense. The Rhodium (sounds like 'radium') also prevents tarnishing.
Other variations in appearance can come from the amount of oxidation coating (usually black) that's applied. That's a Yurman special trick. The coating can chip and it does wear away, but it gives the silver an aged look. All silver oxidizes over time, but it can take many years and it will be uneven. Many silver afficienados buy vintage pieces because of the unique oxidation patterns - so if you have a worn piece don't clean it if you want to sell it.
I've heard some folks swoon over Mexican silver, but my understanding is that you have to be very careful in buying. It's almost all stamped 925, but isn't really. Cheaper metals like copper and tin can be substituted. These are more reactive on skin and can cause green or black marks and/or allergic reactions. I don't think it's possible to have a skin reaction to actual 92.5% sterling silver. I don't think there would be enough copper or tin within the alloy to give you a reaction. But I'm not a dermatologist. I would just be very suspicious of any '925' jewelry that causes a skin reaction. I think it's not correctly marked. There are testing kits available online to test the purity of silver. AND, and this is weird but it works - if you have a sensitive nose, 925 silver has a definate smell. It is distinct and lasts for many years - maybe forever. Smell a piece that you're sure about very closely and deeply. THAT'S the smell of silver. It shouldn't vary if it's 92.5% pure. Silver dealers always smell before they buy. I was skeptical, but it works. I can smell my Yurman bangle now very easily. If Rhodium is applied it will dampen the smell. If you buy 'shiny' silver that you think is rhodium plated - you'll never be sure what's underneath.
I've been told to stick to vintage Mexican pieces sold by reputable dealers in the US and just steer clear of new pieces in tourist areas in Mexico, including more expense pieces.
White gold is only about 52.5% gold. The rest is nickel and a few other metals - actually a whole soup of other metals. The rhodium plating is what gives it the whiteness. Unplated white gold is a white-yellow color, that I prefer to the plated shiny white. It's very rich and warm.
Does that help?
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