If I were you I would buy a perfect lab diamond and sell it at that pawn shop as a flawless diamond mined in South Africa.
If he cannot tell silver, he surely cannot tell the difference between manmade and real diamonds!!

If I were you I would buy a perfect lab diamond and sell it at that pawn shop as a flawless diamond mined in South Africa.
If he cannot tell silver, he surely cannot tell the difference between manmade and real diamonds!!

Actually, no one would really be able to tell the difference between natural diamond and a man made one, if the maker of the man made stone did not add a chemical marker.
I had the opportunity to work at a place that made industrial diamond drill bits (Polycrystalline Diamond Cutters ) for drilling in the oil and natural gas (and impact bits for mining applications as well).
One interesting thing that my company did was to take natural stones and run them under a million + pounds per square inch, and take them up to 2300 degrees, and change the color. If they have the right trace elements in them. I have held in my hand, a 48 carat cut weight stone in my hand. I also ran one stone on my machine and had a 16 carat stone fall to pieces in my hand after the press run (would have been nice if the guys I was running the stone for would have told me it already had the stress lines in it, and they were expecting it to fracture).
Anyway, it is only a matter of time, until there is a flood of man made diamonds on the market with no way to tell them apart from natural stones. The only thing keeping manufacturers from making these stones (aside from the initial very high cost of the equipment, and having a ready market for "undocumented stones") is their own honesty.
My company did not make stones, as the profit margin was lower. The runs for changing the color were very very profitable.
Interestingly enough, you can also make rubies and other stones that are indistinguishable from natural stones (again depending entirely on the makers honesty in adding the trace chemical that makes them readily identifiable as man made).
As to the original post, and the discussion of the "screwing the pawn shop". I have had many dealings with a pawn shop. The same place I bough about 8 or so fire arms, and all kinds of ammo, clips, holsters, etc. After all that, the place still treated me like dirt. Acted like I was bothering them.
Pawn shops are in the business of knowing what the value of their stuff is. They know what they paid for something, and if they don't, it is not my obligation to let them know.
He asked the guy if the price listed the item was what he really wanted to sell it for. I look at pawn shops as garage sales. It is their stuff, and they are selling it for what they want.