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........ we can indeed tell if the object is copper, aluminum, etc. within some degree of accuracy.
All of this is sort of a moot point anyway, because most detectorists bulk at paying out $600, much less $6,000! ..........
On the contrary, if there WERE any machine which could tell actual composition (as opposed to merely conductivity), and do it through the impediment of soil, you would have guys mortgaging their homes to get ahold of it! If, as you say, any device can tell copper, vs aluminum, vs gold, etc... on a size-per-size relative basis (so it's not just conductivity), then think of it: Any of us could go to junky parks, and dig *just* the gold. I'd gladly pay $100k for that. And then just spend my days traipsing through junky parks which for the past 35 yrs. md'rs have gone through disc'ing out the foil, tabs, etc.... And even if I only got 3 or 4 signals a day, if all were gold (by virtue of the technology you claim exists), then the machine would pay for itself.
Hence, no, I don't think such a machine exists, at any price that can do it through ground, and is available for us hobbyists.
There WAS a machine made for the military, that I read about in the early 1990s . It's purpose was to x-ray objects to tell .... yes ... their composition. I think it was more along the lines of experimentation for use to determine explosives, for example. Or pollutants in the case of landfills. The article showed the device mounted on a bobcat tractor, with the operator wearing a lead suit! And aiming it at a concrete block which had materials encased on the inside. The device, yes, could tell the operator that it was xx% tin, XX% lead, XX% this, XX% that, and so forth.
So I sent a letter to the fellow, asking if such a device could tell aluminam apart from gold, on a size-per-size relative basis. I explained in the letter the dilema that hobbyist md'rs face, that aluminum and gold share the same conductivity, on a size-per-size basis. The author of the article actually called me ! After a bunch of talk, he confirmed, that ... yes: If a person had a pulltab that read a certain way on a metal detector (a coord. on our conductive scales/cross-hairs), and a gold ring that read EXACTLY at that same coordinate on the detectors conductive scales, that ... yes, the machine he wrote about would indeed know which was gold, and which was aluminum. Because it was reading actual composition, NOT conductivity.
But as I say, this was avalable at no cost, to anyone, and was strictly military experimental. That was back in the late '80s or early '90s. Who knows what's gone on since then (as far as size and cost). But do you really think you're going to wear lead suits, and get govt. clearances for emittances of whatever it's putting out, etc... ?
As for the shape showing technology, glad you concur that it's of little use to hobbyist. I find it hard to believe that the one your OKM machine can tell the difference between a square and a round object, when you're talking postage stamp/coin sized objects. But even if that were true, I have my doubts that even that amount of resolution will help us. I mean, a tab and a gold ring *still* have the same shape. And a foil wad and a silver dime can have identical shapes. And heaven help you if the item is tilted even to a fraction of a degree! In other words, rings would never, in real world, always be lying flat, for instance. And the moment a coin or ring tilts, you can kiss shape showing good bye anyhow. But I don't think they have the resolution anyhow EVEN if lying flat. It's more like inch-big pixels, if I'm not mistaken. And even THEN, objects you would THINK would have distinct shapes (a horse-shoe for instance) are still nothing more than a "blotch of messy pixels".