My dad made jewelry; really made it in as cutting and polishing the stones, making wax casts for the setting/band, melting gold, using a centrifugal caster to finish the job, and buffing wheels to polish it out. He passed away before I was engaged, but taught me enough to know what to look for. Guess where the engagement ring, and darn near every other piece of jewelry my now ex-wife got from me came from... Pawn shops.
Yep, and when I was in the army, the pickins were REAL good around post, where there were TONS of competing shops, and TONS of jewelry to go over to find a beauty. Sure, some folks might sneer at a "used" diamond/pawnshop stigma; fine, let em pay the xxx% markup, and many times settle for a crappy stone. People would ask me to help them find a stone with some sort of complicated "4C's" spreadsheet, and various "certificates" that vary, making a clear valuation difficult... Forget it. I would tell them a diamond to worn as jewelry has but one purpose; to be beautiful. No women I ever met would rather have a large, dull, brown, included stone, with a piss poor cut. And please, no "Kentucky clusters" either. So much better to buy "used", and get a stone that has great fire, clarity, and properly cut facets.
Also, you don't have to be an expert; just get yourself a quality 10x or greater loupe, and look at the stones in differing light to see how it looks. Also, I love pawnshops because you are fully expected to haggle/dicker, and even have some fun with it. Can even have them just sell you the stone, and bargain how much they will knock off with them keeping the precious metal setting/band. Have fun, take your time, and keep your options open. Good luck!