Very good post Charlie. And I would add more woes to our dilema: there are alloys that affect resultant conductivity. To simply say "gold is a low conductor" for instance, is a loaded statement. Because in fact, gold in its pure form (24k) is an excellent conductor! If you had a small gold ring, made of 24k pure gold, believe me, it would read up at penny/dime (high conductor). But the same size ring, made of 14k gold, would read down at tab, nickel, or whatever (low conductor). The reason for the lowering of conductivity, is the alloys added to gold, to harden it up into useable jewelry functions.
And size plays in to the equation too. For example: Aluminum may be a low conductor ..... SIZE FOR SIZE! But if you took an entire aluminum can and waved it in front of your coil, you would get a penny or quarter (high conductor) signal, right? But why is that? A tab is aluminum, and an entire aluminum can is aluminum, so why did the TID's read different in each case? Because size plays into it.
Thus the harsh cruel world of metal detecting disc.

If they ever invent a machine that can tell actual composition, and not just conductivity, we're all going to be digging gold rings till our arms fall off, eh?