Rare resource

A rare resource, also unofficially known as precious metal, is a type of resource in which they are only found on specific islands. There are three rare resources: marble, iron, and gold, so you can only supply your city with the one resource that happens to be minable on your island. As such, you will have to attain the two other rare resources either from purchasing them from the market or awarding yourself through quests and attacking other cities. You can figure out what the rare resource is on an island as an icon on the map.

Rare Resource Building
The rare resource building is a structure that you build in your city to attain the rare resource. It is the only building that can change, island to island. The rare resource structure changes its appearance and name, depending on the island, but they all do the same thing - mine your island's rare resource. Below is a list of the three buildings, which are associated with the resource. You cannot build more than one type in a city.
 * Blacksmith - attains iron.
 * Marble Quarry - attains marble.
 * Gold Mine - attains gold.