farmer85j, you can use a cheap thin filigree gold ring that you have cut up and pounded into a Small ball/space. It will simulate a natural nugget well enough for testing and setting your detector with. I would cut the band off of the Filigree and cut it into a 5 gr piece and then smash up the Filigree part into a small flattened ball. You can buy this from any Yard Sale costume jewelry box for a couple of bucks. Heavy Gold Plated will work just fine as the gold plating on the outside is what will be seen by the signal from the detector.
If you can find a fine gold chain, I have used it to begin with, take it and mix up some two part 5 min epoxy and put the chain into a small 3/8" dia. plastic lid, after you have put the epoxy in and let it set some. then put the chain in and press it down into the epoxy. When the epoxy has hardened you can take the outside plastic off and now you have an indestructible test nugget. I make different sizes of these and sometimes bury them at different depths to tune my machine to the Ground I'm hunting.