Thanks, for your response, I did purchase the stone from BTD.
The vinegar & rubbing alcohol ideas have definitely improved the store- but still not a 100%.
Going forward, once this issue is resolved. How can I prevent a similar situation from occurring?
Ok, if you are seeing improvement with the vinegar, then the issue is as I thought, namely hard water build up.
The reason it is happening on the top and not the bottom as much is simply because water will pool on the top of the stone due to gravity.
As the water evaporates, it leaves behind a very thin film of minerals...and do this over and over and eventually it builds up thick enough to impair the optics.
Anyway, the fix is to keep going with the vinegar as JCent noted...it's a slow process b/c you are literally undoing layer after layer of mineral build up.
Assuming this is not micropave and you have a good ultrasonic (not the $20 Ebay types that do nothing but vibrate the water a bit), then get some distilled water (not bottled, but distilled) and make a 50/50 mixture of distilled water and vinegar, and let it run in that in the ultrasonic.
The cavitation will help loosen the top layer of the deposits and the vinegar will react with it and pull off a layer at a time.
Distilled water is important because it has no mineral content and thus won't worsen the situation. Otherwise, using normal tap water is just supplying more building blocks to the deposits that are already there.
To avoid in the future:
You'll need to basically try and prevent it from building up in the first place and that will likely mean at least a weekly soak in a mix of distilled water and vinegar to breakup and remove mineral deposits before they become noticeable.
The only real fix is otherwise to only wash your ring in distilled water, but that doesn't seem too realistic
Fyi - the gemcare won't help much with built up mineral/hard water deposits. It's a good cleaner, but it is more for organics like grease/oil. There is likely some grease/oil mingled in with the mineral deposits, but the mineral layers are the hardest part to remove.
Anyway, at least we've likely identified the problem and I think with a bit of work, you should be able to undo the deposits and get it back to it's original form at home.
Hope that helps