What you need to know about treasures:
- Talk with people in the taverns, coz sometimes you will find someone selling a treasure map(it seems to happen more often when you have assigned for 'search item' job in a town hall).
- Each map costs 2000 money.
- You can sell a treasure in a town hall that is looking for them.
- I dont know if its a bug or what, but when you have a treasure sometimes a guy in the tavern asks "Hey, is this the dark crystal(or any other treasure)? What about 10k for it?". You can sell for him, but the funny thing is that you dont have the dark crystal. It happened a lot to me.
- Finding treasures increase your explorer level and fame.
How to find a treasure:
1- Buy a map. You will receive something like this:

You can visualize it when you are at sea.
2- Well, now you should start navigating near the coast of the continent and islands to find a place exactly as in the map. The above map is for the ancient manuscript, near Triaste.
http://img146.imageshack.us/img146/3644/ancientmanuscriptkt4.jpg
3- Aproach the coast where the X is marked on the map, at low speed (or you can get stuck) then land and click search. Voilá!
http://img146.imageshack.us/img146/479/landingrh7.jpg
These are the treasures that ive found already:

Yellow(near Libreville) - Golden Tear
Red(near Santo Guanta) - Saruman Orb
White(Port Halcourt) - Diamond Eyes
Orange(Aalborg) - Malachite Box
Green(Vlissingen) - Jade Jewelbox
Dark Blue(between Fortalesa and Puerto Caballo) - Garnet Brooch
plus the ancient manuscript.
If you have found different treasures, post here plz.