Quite a lot of the time I'm able to get large parts of seeds by hitting the shell against the ground. This causes bigger cracks in the shell than the seed, I can then pry off the outer layer. However, I've never been able to get a whole seed this way.
The only other ways I can think of would be dissolving it in an acid or using abrasion, but I do not know if these would work.
What is the best method of removing the dark outer shell from a coconut without damaging the inner white seed?