Treasure Hall is a place where you can store your gems and no one will be able to steal them. However, you won't be able to sell them or perform rituals with these gems either.
To save your gem in the Treasure Hall, you need to spend some Emeralds. Emeralds are the resource that can be obtained by performing rituals. The number of Emeralds you receive depends on the value of the gem that you get when you complete a ritual. The more valuable the gem, the more Emeralds you will receive.
Each time you save a gem in the Treasure Hall, you increase your Saved Rating. When the Rating is high enough, you will get a Magical Sphere that contains Unique gems. If you save gems with more than 1 000 000 value, you will get a bonus Sphere.
You can find all the gained Unique Gems in the Collection. By the way, when you add a Huge Gem to your Treasure hall, it will also be counted in the Collection.
Beginners who have just installed and started the game will gain access to the Treasure Hall when they move to the third dungeon.