I'm at a loss to id your seed.* At first I thought it might be the seed from a cone such as a cycad would produce, but I can't find anything that looks like that.* I've looked through pics of many tropical seeds of trees and didn't find a match.* I'm asking another poster from Hawaii to take a look.* Maybe she can id it.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.