So, here's an auction result for a similar sized stone: https://www.liveauctioneers.com/item/20832576_platinum-3-stone-ring-w-5ct-green-diamond-center

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It's a bit more extravagant than your ring and the stone is bigger. However, to me, that says that yours is easily worth 30k+ for just that center stone by itself. I feel like the jeweler that told you 8k was either ignorant or was trying to pull a fast one on you. The latter scenario happens all of the time, unfortunately.

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It would probably be worth your while to have the diamond reset in another setting that looks more interesting possibly with some matching accent stones. That would make the stone more appealing to someone willing to spend that kind of money on a ring like that.