I can't address whether they're natural (not-cultured), but if you believe they are cultured, there may be another angle that would make you equally or more happy IF you are able to back out of that purchase.
The more commonly mentioned vendors that are often mentioned here have akoyas in a 6.5-7.0mm size, quality AA or AA+ (based on your statement that some of the pearls have blemishes), 18" length, for less than $500. 18" AAA freshwaters seem to be about half the price of the akoyas - under $200 for 6.5-7.0mm. You'd be guaranteed to know what you're getting that way.
That said, if your strand is akoya I imagine it would appraise for what you paid, but others members here are more knowledgeable than I on that topic and I'm confident some will offer their expertise here.
Good luck! Kudos to your Hubbie for the gift!