I found some stuff to explain Magnezone's evolution process:
Magnezone can only evolve in a magnetic field that can't be recreated in a lab. Field strength is measured in Tesla units. The earth's average field strength varies from 0.000031 to 0.000065 Teslas depending on the latitude.
Now, there are four places where Magnezone can evolve in the Pokémon world:
1.Mt. Coronet: The highest mountain in Sinnoh and the location of the Spear Pillar, home of the portal to various dimensions.
2.Chargestone Cave: A cave in Western Unova that is full of glowing, floating stones.
3.Kalos Route 13: A barren wasteland that is the site of the manmade Kalos power plant.
4.New Mauville: Another man-made powerplant located under Mauville City in Hoenn.
We know all of them have some sort of strong magnetic field. For both power plants, the source is relatively obvious. Any moving charge creates a magnetic field, whether it's an electron around a nucleus, or a current through a wire. These power plants must manufacture and distribute massive amounts of electricity, which generate a massive magnetic field appropriately.
Chargestone Cave appears to be a deposit of diamagnetic superconductors. Basically, this allows the stones to easily float, just like Magnezone itself does. As for Mt. Coronet, it's probably either a strong magnetic pole, contains lots of ferromagnetic ores, or perhaps theres something about Arceus that generates a big magnetic field.
In any case, all four locations have a Magnetic field in the order of around 1 Tesla. About 10 Teslas will kill a person, and since the areas are frequented by trainers, it's definitely closer to 1.
Now, I am lead to believe that laboratories can easily produce magnetic fields that are larger then anything that naturally occurs in our solar system. The largest ever lab-generated field was 45 Teslas, while the largest ever lab-generated pulse field was 730 Teslas, which destroyed the equipment used. If Magnetons evolution was based on field strength alone, theres no reason why scientists could not recreate it in a laboratory setting.
But because Magnezones evolution is not artificially possible, there has to be something else unique about these sites that triggers Magnetons evolution. Based on my understanding of what the internet tells me, I think the answer lies in how Magnetons magnets interact with the environment around it; not the other way around.
Every material, I'm told, has a quantity called magnetic moment, which determines the magnetic force it will feel. During the experiment performed by the scientists mentioned in Magnezone's dex entries, they likely isolated and insulated their experiment from any other factors. So while the strong magnetic field could interact with Magneton, Magnetons own magnets did not act on the thing generating the magnetic field.
I am told that essentially, Magnetons own magnetic field interacting with its environment around it (the thing thats generating the field) would take the already large magnetic field and make it into a changing magnetic field. Changing magnetic fields apparently have lots of interesting effects that stationary fields do not. If a field is changing fast enough, it can supposedly even have the same effect as bombarding you with radiation.
Radiation is what causes Magneton to evolve, or rather, to mutate, into Magnezone. This fits in well, since Magnezone is supposed to be a Magneton that has 'melted' together. Magnezone gets the radiation energy it needs to evolve from a strong, changing magnetic field. It is not re-creatable in a lab setting because Magnetons own field is not allowed to interact with the equipment generating the external one; which would likely harm the equipment anyways. We know that Magnezone's magnetic field is sufficient to casually destroy mainframes and supercomputers, as stated by the dex.
So, best stay away from Magnezone when it's evolving, because I understand that being exposed to the radiation of a rapidly changing magnetic field is akin to being stuck in a microwave!