How upsetting, firegypsy. Sorry this has happened.
I think the first thing to check is the law in the state where you live. In the UK, we have the Sale of Goods Act, which, along with common law rules covers pretty much every transaction. I would imagine you have something similar locally. Check specifically if second hand or estate goods are exempted from statute and what the position is for them.
Usually in a retail store, you have the right to rely on the description and verification of goods by the seller, who is presumed to have sufficient expertise. You aren't expected to be an expert and you aren't required to have the seller's claims verified by independent appraisal for the transaction to be validated. If it's sold as 18k, it needs to be 18k and you're entitled to rely on that (ie it isn't your fault for not checking / knowing). In that sense, caveat emptor does not apply.
Months later isn't a problem, as far as I can see, since there was no time-limited action on which the contract was dependant, eg you didn't say you'd have it appraised in 7 days or whatever. Within a year is not an unreasonably time frame to discover that goods are not as represented at the point of sale. Again, check your state's legal position.
However, without a receipt, I think the problem will be that you have no real proof that a) you bought it from him and b) that it was misrepresented, either wilfully or carelessly.
I think your best recourse is to know the local laws, and to go with that information in mind, to speak politely to the seller, to ask how he would like to resolve the situation.
If you've shopped with him before and trust him, there is a possibility that it is a genuine mistake and he will be keen to make it right.
I'd stick to DBL in future (but then I'm hopelessly biased).