This would be a simple plugin where players could convert any emerald in the hand or inventory to currency. There is already something like this, but it uses gold and is not configurable: http://dev.bukkit.org/bukkit-plugins/ceconomy
I'm aware of this plugin, but I don't think it has the ability to convert emeralds to currency, it simply uses them. What i mean is that it would take emeralds (or another item) and convert them to a currency that another plugin would handle. Similar to the example I left above.
How would you like this plugin to work? Should the user type a command with it in his hand, or should he right click while holding an emerald, or what? Please give more details. Also please add any commands/permissions you would like this plugin to have. ContributeCraft
They could type a command that would exchange all emeralds in their inventory for currency, and also a specific number of currency back to emeralds into their inventory. I didn't have any commands/permissions in mind, but the config.yml could allow alternate items than emerald too.
Someone just pmed me with this today from another forum, hope you didn't start yet! http://dev.bukkit.org/bukkit-plugins/goldtocash/ It allows the conversion of multiple items. Thanks xBlack!