Not necessarily messed up when they give you the reason as to why they won't take it.
Some nyrea carry those gems on them, so it's relatively not as valuable to them as it is to a vendor who can possibly trade gems with other merchants for more relatively valuable things. Platinum is extremely rare in their environment compared to the gemstones already found on Myrellion, so it would make sense that they will allow entry for that instead.
Wait why isn't fighting them an answer for you? Fighting them has zero repercussions(unless I'm very mistaken) and it's a -relatively- easy fight if you can beat the queen of the deeps.