Just in response to Danlmer, as his post was moved to the insufficient information sub-forum so I can't reply there;
I just popped out of my MH to Arbitrix, the Gobbie in charge of the box in Windurst Walls. I was able to trade my Gem of the West without issue, so this sounds like it must be some kind of bug. Gem of the West shouldn't be grayed out on any trade window - you should be able to choose it in a trade to any NPC (of course will have no effect to any other NPC).
If you still have the issue, it might be worth doing a File Check from the POL Viewer, as the only reason I can imagine this happening is a corrupted flag in the .dat file that contains the item database.
Otherwise, Bug Reports on the forums need to be filled out as per the sticky post here for them to be referred up to the devs.
Hope it gets sorted!