All this info is based on my experiences using a neodymium block magnet. I have not tried it with lesser strength magnets.
A strong magnet can help to identify silver based on its diamagnetic properties. Moving a strong magnet very closely over the surface of a silver dollar will cause it to shift slightly. It's most noticeable if you hold the silver dollar in your hand so you can feel the slight interaction. I have used this to buy a silver medal at a flea market (for much less than its real value, I've got to brag.) The effect is slight, but has been used to my advantage at least once.
I assumed this was unique to silver, but it turns out the magnetic response of non-ferrous metals is not as simple as my ignorance would imply.
For some reason, while recently sorting aluminum heat sinks I passed a magnet very closely over the thicker solid surface of the aluminum and was surprised at the strong interaction. It is noticeable with both cast and extruded.
Not really sure how to use this information yet, so I'm just throwing it out there for ideas. One area I think I might be able to use it is to identify brass vs. colored cast hardware. Flea market and yard sellers usually don't like their items hit with a grinder before I buy their stuff. Although I have a lot of experimenting to do, it seems that cast aluminum (paramagnetic) interacts more strongly than brass (copper and zinc are both diamagnetic) to the magnet. I need to dig through my brass box and test different sized pieces. It's immediately obvious that thicker pieces of metal interact more strongly than thinner pieces. I suspect the presence of ferrous metals would completely overwhelm the ability to detect the paramagnetic interactions.
Anybody else ever used this? If nothing else, I have a new cool thing to keep the grandkids busy for 3 minutes while working in the shop.
Just a bit more info:
Magnetic Type for all the elements in the Periodic Table
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