I'm afraid that Ringarin is wrong on this one.
1 - No you cannot harvest coins. This is due to the fact that the HARVEST city action only allows you to get one RESOURCE (wheat, incense, silk or iron) available to the city. Coins are not considered to be resources. You simply have one more coin on your civ sheet dial as long as you have a scout standing on it or a city whith it in its outskirts. If, for whatever reason, the scout moves away (for example if you need something else more or it is killed) from the printed coin or the city is destroyed, your coin count immediatley goes down by one.
2 - See "Map Tile and Icon Breakdown" on page 7 of the base rulebook for a list of what is considered resources and what is not. Also see "Coins on the Map", bottom left of page 26.