I disagree. Diamonds aren't really that big of a deal, especially since the advent of mending means all your tools will last forever. You basically need less than one stack and you are set for life (unless you tend to die and lose your items to despawn regularly). So diamonds aren't really that valuable. However sea lanterns are pretty valuable, they are difficult to get for the average person. You either have to raid a monument (which if there are any on the server I doubt they haven't been raided) or build a farm for them, so they aren't easy to get. Diamonds you can just dig into the ground and the law of averages says you will eventually get diamonds. I think the price of 25 mtc for a diamond and 7 mtc for a sea lantern is pretty balanced. Besides if it wasn't, as you said yourself, the market would adjust itself to to fix. All that being said, I am pretty new to the server and might be missing something, but the economy seems to be fine to me.