Welcome to the Upper Middle Age bracket! I got some doubts that the big stone would be a diamond in that mount.. it would be a whopper if so. Not even sure it was made to hold another stone. You might run it by a jeweler for an opinion on whether it was really made to hold another rock. If so, it might have been an emerald or ruby or maybe a garnet or even amethyst. You have a problem I have thought about but never actually had yet. My projected-imaginary search design is 1, Make sure I can get back to the Exact Location of the find, 2. Go back with a small, clean up-sized gold pan and a bucket of water so I can wash the mud & dust off the surrounding material that I have put in the pan, then carefully thumb through the suitable small pebbles that have been cleaned. A cut stone should show up well under a little water and good light. 3. Alternatively, I could do roughly the same thing with a little frame with some window screen tacked over it. The parks people probably won't crab too much if you put all the material back in the hole. If somebody wants to know WTF I am doing, I can come up with some yarn that amounts to the exact truth of the matter. Sounds like a fair weather project. In your case, take your gang with you - or at lest one armed guard to watch your back. Good luck.