Gold (Au) is the 79th element in the periodic table, it's a very (the most, by pure elements) malleable, soft metal that has excellent biocompatibility properties. It's also worth a lot, and has better value than being used as a coating for a magnet to be implanted in vivo.
Gold IS technically a viable option for a magnet coating, though it is among the most fragile coatings you could put on it, and are prone to higher than normal failure rates.
Coating Gold is able to be chemically electroplated over surfaces to form thin coatings, which in theory, should be completely biosafe.
The reason this isn't suggested is because of gold's inherent malleability and lack of structural strength. It is very easy to damage during implantation, in vivo by friction from movement against tissues or any debris, or even from force applied.
It is also vulnerable to 'pin-holeing', as any type of thin coating is. In any case with any coating, they should always be tested for failures before use.
Gold makes a wonderful compliment to work with Parylene (Bio-compatible Polymer) as a base coating.