Dover, You wrote, among many other things, "The piezoelectric properties of a quartz crystal make it usable as a resonator." With all due respect,and I do respect you, as I understand it, a quartz crystal (or any crystal) when energized by electricity will indeed resonate at a unique frequency. That's what makes it so useful in servo and clock mechanisms. Along with this resonance, RFI is emitted. In this case, as I see it, RF noise and EM noise would be synonymous, but,when we just mount a crystal out in space on a cartridge or tonearm there is no electrical energy source to set the crystal into resonating, and I don't know how "the EMI emission from the crystal can alter the behaviour of other EMI floating around in the vicinity", even if it were emitting EMI/RFI. These frequencies don't shut each other down or interfere with each other very much except to re-enforce each other. Otherwise, we would not have radio stations broadcasting on adjacent bands that don't differ much in frequency.
And, as Geoff pointed out, I don't think this is "piezo" effect, for which pressure on the crystal is required.
Wasn't there also a ZYX cartridge that had a spherical sapphire-colored something mounted forward of the body? Was/is that supposed to be a crystal?
I am beginning to think I have to try this stuff, even though I would say the jury is out on how crystals may work.