Here is some information that could be along your frequency lines.
Both gold and silver are diamagnetic, meaning they are slightly repelled by magnets.
Chance can easily detect this ceramic plate that has 24k gold on it fired in a kiln.
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Bury that at 6 inches. No problem. He triggers right on top of it.
This plate, on the other hand is 24k gold plated over some other metal.
Not sure what the metal is but it is magnetic. A magnet will stick to the plate.
It's an antique and is marked as 24k gold plated by a reputable company.
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Chance cannot scent detect this plate.
It can be in open air, right in front of him. He will not trigger.
Acts like it is just another meaningless object.
He triggers on any gold from 10k to 24k.
Jewelry, Coins, nuggets, bars. gold in quartz, gold plated quarters.
examples.
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Any gold or silver he has been able to do.
Except that gold-plated plate that is magnetic.
So, something else is going on besides just a sulfide scent.
The gold-plated plate should have the same scent as everything else.
But it is totally invisible to him as gold.
This may fall in line with your frequency concept since it is magnetic. I'm not sure.
But something for you to consider.
Why can all other gold and silver be detected except the one thing, that has a very large surface area and should be emitting tons gold-sulfide odor, not be detected? Except there is also a magnetic component involved with this item. I'm almost positive the company that manufactured the plate is legit and it is really 24k gold plated. The only difference is the material it is plated to is magnetic.