Is this a fair offer?

If that is a dealer giving you that offer, they are going to low ball you and hope you do not investigate further (IMO). I would think it would be worth getting an independent appraisal done to get an accurate price at fair market value.

It sounds consistent with what I've seen similar stones sell for on the secondary market.

Rule #1, 2, and 3 get an independent appraisal and never ever sell to the same person offering you an appraisal. This is just so basic. Once you do that you can determine what similar stones sell for on the secondary market because you'll have a more accurate assessment of what your stone really is!

In my area, an independent appraisal is $125. IMO, worth the investment. If you're looking to sell, be sure to specify that you're looking for fair market value, not insurance value.

It's easy enough to collect bids from several other buyers and see how they compare to one another. In particular, pay attention to the grading. The difference between an I and a K is huge in terms of price and surprisingly subtle in terms of the look. We, of course, have no way of knowing what the facts are but YOU need to decide. Without this you have nothing. Hire an independent appraiser or send it in to a lab. Basing your grading on a bid from someone who wants to buy it and pricing based on advice from an Internet forum is a risky strategy although it's hard to beat the price of the service. Averaging bids from several potential buyers is better but can be problematic depending on who you choose as advisors.

Get some more bids, see how it all stacks up. The more work and investment you put in the more return you will get though -- a lab report, or independent appraisal from a respected authority, will go a long way.