I was just posted the theory for the black sand switch ability to deal with ground, and not it's detailed, or actual function. That being, that it is supposed to "help compensate" for mineralized ground, and improve performance with out having to adjust the actual ground balance trimmer. But, then again, I always looked at ground elimination, and rejection as discriminating out the ground minerals, using discrimination, even though the ground elimination, and user discrimination are separate on the circuit.
I made the statement about the nickle, to illustrate the fact that it does change the discrimination range, but after re reading the original post, I should have said to use a very small ring, since he was mainly interested in the ability to detect jewelry.
Looking back I suppose I should have explained, using the TR type detector, discrimination, and ground exclusion balance adjustment better, and it's effect on detector response, and performance, especially dealing with low conductive "an items ability to conduct electrical current",items and that an improperly adjusted ground balance to far positive or neutral, can affect not only your "all metal mode", be it motion or non motion, but your target ID, discrimination, depth ability and sensitivity. The detectors ability to transmit and receive, is affected by, any conductive, ground minerals, EMI,natural occurring electrolytes ie, wet salt in sand, and other phenomena. And the difference in discrimination, and ranges, a full range discrimination, would actually be able to accept all conductive items at it's lowest setting and reject all conductive items at it's highest setting. In reality a metal detector is designed by it's very nature to detect metal, as such, any elimination, or discrimination, of one type of metal, it's oxides etc. will affect it's ability to detect other metals, when discriminating out a conductive item, you are limiting, blocking, or altering, some of the receive signal to the operator indicator, ie speaker, and that cannot be avoided, this includes eliminating any conductive item, be it ground minerals, or highly conductive items, ie, US silver dollar. Also, when using your detector remember that low conductive items in large mass ie, a man hole cover, may react as a higher conductive item due to it's ability to reflect a large amount of the transmitted signal. Actuall TR metal detecting discrimination circuits, "to the best of my knowledge" deal within a range of conductivity, So, you may see a discrimination control marked, ferrous, "contains or made of iron" or, non ferrous "is not made of, or contains iron", the ferrous item will generally have a lower conductivity than a non ferrous item.
Anyhow, you did a great job of explaining the features of the Classic series, guess I was one of those old "not so knowledgeable dealers"
HH.James