Unlocking chests - maybe, the bigger chest the better.Investing in guild buildings - almost never, coins can do that.Resting characters - never, time provides that.Rushing crafts or quests - never, time provides those.Decorations and customizations - never, they seem to provide no in-game bonus.Stands and resource bins - never, there are coin options for all of them.Upgrading an item to a higher rarity - almost never worth it, unless there's a way to get 0% chance rate of breaking (which I haven't seen so far in this game).
I'm level 32 and generally want to get an in-game bonus that makes me stronger. I've been getting diamonds through playing and I've been wondering what best to do with them.
