The other day I found a neckless that was so encrusted and black I started to toss it. The links were almost welded together with encrustation. It took quiet a bit of manipulation to get it freed up. I haven't attempted to clean it yet but it has been in the sand so long that it is pitted and I can't see a hallmark of any kind. It was approx. 10 in deep and actually gave a distinct high tone and vdi of 26-28.