I want to reply to that, but not in that thread.
Carrier frequency has nothing to do with available bandwidth. Bandwidth is directly related to bandwidth, so the faster you want to transmit, the bigger the chunk of the spectrum that you require. The relationship between maximum data transfer bandwidth and RF bandwidth is defined by the Shannon–Hartley theorem.
But as expected, Amal is right and that magnetometer thing will pick up a lot of crap from the environment… In fact, you can play around with a plain old compass and it’ll be affected by things like rebar, screws, nails, furniture parts, some keychains, etc…
And you are indeed correct about the size of the required antenna for low frequency stuff.
It’s also worth mentioning that some metal detectors tend to struggle with iron as it has a higher resistance than other more valuable metals and this makes inducing eddy currents a bit harder. And IIRC, some metal detectors use the phase of the returned signal or something along those lines to avoid detecting ferromagnetic metals as stuff made out of iron is rarely valuable.
Besides, just like Amal said, small bits of metal are very hard to detect.