As someone who was there seeing the very first TID's introduced to the market in 1983-ish to the present, let me put it to you this way:
The TID's on today's top-end machines, have not only simple up/down axis, but left right axis, graphs, tones, etc.... of any mix and match a person desires. Over the years, there have been many who have thought and tried to dicipher a pattern to the possible differences of where gold jewelry reads/sounds, verses where aluminum junk reads/sounds. It's tempting to think that there MUST be a pattern or consistency, because afterall, there's oodles of coordinates, sounds, consistencies of bounce, etc... that can occur for the myriad of targets sampled.
The end result, after over 25 yrs. of TID development, is that there is no magic formula, either by sounds, tones, TIDs, graphs, grids, etc... that can tell you the difference between gold and aluminum. This is because there is infinate sizes, shapes, weights, and karots of gold jewelry. And there is infinate possible sizes, weights, shapes, of aluminum shrapnel, bent tabs, foil wad thicknesses, etc.... And unfortunately, aluminum and gold share the same conductivity.
If anyone tells you differently (that gold has a certain "sound" or "tone" or "smoothness" or "repeatability", etc...), then do this: Quickly take that person out to the nearest blighted inner-city urban park, and turn them loose. See how much gold they dig up, while leaving most aluminum in the ground. I think you will see them abandon their theories
Yes, on an individual basis, you can see/hear the difference between a given gold ring, and a given foil wad. Granted. But so too is there a "difference" between each foil wad too. And so too is there a difference between each piece of gold jewelry too. And if you ever dig a gold ring, from amidst a sea of tabs and wads, you will undoubedly think "this one sounded different!", and set about trying to determine how this sounded "different". But all that is is mind tricks of selective memory. Every time we stop to dig, we subconsciously think "this one sounds different". But when we pull up a foil wad or tab, we think to ourselves "yeah, come to think of it, it
did sound kind of junky" But when we
finally find a gold item, we selectively remember our premonition and think "aha!! I
knew it sounded different".
It's the same psychology of thinking our dreams come true. We dream hundreds of dreams a night, and promptly forget them all upon waking up. But if coincidentally a single one comes true (you hear that song on the radio you dreamed about), you remember just that one dream, and think "gee, I'm psychic!". Same trick going on.
or a number 
