Protective insulation for magnet installed in earth
Assuming you just want a fairly thin layer of protection and it won't physically get knocked around too much the same sort of conformal coating often used on PCBs might be worth considering. Here's an example from Element 14:

Electrolube - EFSC15ML - Conformal silicone coating

You can buy it in aerosol cans as well but brushing on or dipping will probably be most convenient for a magnet. As pjc50 mentioned in a comment it's probably also worth considering regular paints. The silicone will give you a small amount of impact resistance but if you don't need any at all I imagine a regular metal primer would be as good as anything.

For something much harder to give additional protection potting mix might be worth considering. It takes quite a while to cure and can be messy to work with but here's an example product:

Robnor - PX804D/BK/050 - Encapsulant, epoxy, flame retardant

That starts off as a fairly think liquid so you'd need to make your own jig to hold the magnet as it cures, which can take a few days when using larger quantities. Maybe you could use a piece of PVC pipe and an end-cap for example, pour in the potting mix and once cured cut off the excess tubing. If you go that way read the instructions and safety precautions carefully, the stuff I've used has noxious and toxic fumes while curing.

Here's an example of what potting mix looks like when it's cured. This was a 9V GPS datalogger I used to sell years ago when they were less ubiquitous and used the potting mix both for physical protection and so that I could use a standard enclosure without the battery shorting anything. The depth of the mix was approximately 20mm and once I took a sledgehammer to a failed unit and it took many hard blows to break it.