Treasures are unique collections that are only available in certain game locations (expeditions). The treasures section only appears at the location where you can gather the required treasures:
 
 
If you tap on the treasures icon, you will see how many previous stones you have already collected:
 
 
Note: you can access the treasures section and check your progress only at the location where you are collecting the treasures. Once you return to your home location, you will no longer see the progress until you return to the expedition with the treasures.

Please also note that the time for collecting treasures is limited. You will receive the reward only if you manage to collect all the treasures.
 
It is important not to get treasures confused with collections that you can access if you tap on the "backpack" icon at your home location. Please refer to this article for more information on the collections.