Most jewelry stores have certified gemologist on staff to check the authenticity of the stone. Many of engagement rings only have 1 diamond to check anyhow!

Nevertheless, you're right about trust being a big issue when you deal with jewelry. I think the large chains (like Kay Jewelers) do offer a 30-day money back guarantee. And wholesale jewelers buy diamonds sight unseen (and leave diamonds to be appraised by the buyers without any collateral!)

All that said, I think if you're talking diamond, the biggest hoax is that: it's not rare at all! It's "scarcity" (and hence high price) is artificial.

It's called counterfeits.

You need to pay an expert ( The guy who sell the rings is not that expert ) to check that every little rocks on the ring is authentical and the only way to do this is to examine each and every little rocks one by one spending sometimes more then 1 hour of miticulous scrutinization. That expert does not work for cheap and you have to take his word for granted or pay a second one.

Let's say you're the shop owner and you take the ring backfrom a customer, you get it checked by a jewel expert who happens to be a scam artist. Both the seller and the scam artist have left the scene of the crime by the time anyone could notice anything.

Jewellers all around the world lose a lot of money that way. Trust is a big issue when you trade with diamonds.