Not sure if this has come up before, or if there is a perfectly good reason why this isn't done, but why not have the same Villager at /warp Donor that does 1 Emerald for 2 Diamonds trade back at the same rate? Might encourage its use as a viable form of economy if it can be more easily traded back down. As it stands, an Emerald is like a $100 bill in a world of gas stations. No one accepts it. Though, in this instance, that's because it is really hard to get traded back to Grass. Like I said, could be a perfectly good reason why this isn't done, but jpyatt messaged me in game asking if I was in the market for one, likely because he got saddled with one somehow.... I'd like to help him, but it would almost feel like a freebie; taking an item for which I don't have much use.... I don't see that doing this would damage the economy in any way. It would just make some trades require less items. 60 Emeralds instead of 120 Diamonds for a Beacon (a cheap one, to be sure) would allow it to be put in a trade chest and traded safely, without having to worry about finding 60 people who want an Emerald for some reason.
