Flake or fleck is anything you have to wet your finger to pick up.
Picker is anything you don't have to wet your finger to pick up (two tries max to pick it up lol) but is just not impressive enough to be called a nugget.
Nugget is anything that you don't have to wet your finger to pick up, makes a nice 'clank' when dropped into a pan, and is impressive. You'll know when it's impressive too, you find yourself saying things like "wha-hoooooooooooooo"!
I used to think anything you found with a metal detector would classify as a nugget but this year I got a Gold Monster. Using the above criteria it finds flakes, pickers and nuggets!