Koetsu cartridges. They must be something special.

I have not heard one, but some say thar Etsuro Urushi cartridges are similar.  The bad news is that they are quite expensive.

Re magnets, bizarrely, they do have a sound. If you listen to some Strat pickups with ceramic magnets vis a  vis those with alnicos, you'll hear it pretty quickly. And its not just down to field strength.

Some of the best midrange you’ll ever hear. Defined, full, smooth, exciting, they open up a whole canvas of sound when compared to other cartridges. This, balanced with thunderous bass and ever so delicate and defined treble make them a joy to listen to for hours requiring no effort from the listener. That’s what makes them special. Just my opinion. 

Yo-yo, so you’ve compared two kinds of magnet set up identically in terms of geometry and field strength and whatever other parameters pertain and compared them on the same Strat ( important to use the same guitar both ways)? And listeners agree the sound is different? (Not to mention we’re talking about a guitar pickup not a cartridge.) even if it’s been done, that doesn’t explain a mechanism.

but what is the possible mechanism, other than the above, mass, field strength, and geometry (meaning how the magnet and coil are placed in space)? Can someone ’splain me?

"Dynamic conditions". The magnetic field varies during playback (however slightly), and supposedly the different magnet types have different behaviors in this respect. No that’s not a satisfactory scientific / engineering explanation, but this is sort of analogous to arguing around "all amps measuring below 0.01% THD (or whatever) must sound the same". Now the anecdotes, for which I have a unique one to this topic:

I have 2 Onyxs, one an old 80s/90s short body with samarium-cobalt magnet. I acquired it with original parts, but worn stylus. It sounded absolutely glorious for about 70% or the record, before the stylus showed its age towards inner grooves. I had this rebuilt by Koetsu in 2018, specifically instructing them to keep the original magnet (because I thought this was part of its "magic"). The other is a circa 2000 Onyx Platinum that I’ve heard both in original condition, and after its own 2018 Koetsu rebuild.

I draw the following observations from these examples:

• The Onyx Plantinum (both incarnations) sounds very similar to other platinum models (Coral, Jade, Blue Lace, RSP), with slight differences among them. The Blue Lace is the best one (for some reason), but they are all of similar character.
• The old Onyx in original condition had the most euphonic and least accurate sound. It was glorious, however. Just a pure music machine.
• The new coils & parts (other than SC magnet) put into the Onyx have a brighter, more extended, dynamic and "modern" sound compared to the old coils & parts. Apparently this is "tempered" by the smoother sounding platinum magnets, because that’s its only meaningful difference now from the other Onyx.
• The SC magnet gives about 2 - 3dB higher signal output versus the Platinum magnet of same size. I guess we’d need larger Platinum magnets to compare them at equal-output levels, but that will never happen.
• In hindsight, I would’ve asked for a new platinum magnet with the old Onyx SC rebuild. The platinum magnet and new coils appear to be properly "balanced" for each other.

IIRC people still chase old Lyra Parnassus cartridges to use as a "donor" for its platinum magnet to use in a new Olympus build.