Treasure Hall is a place where you can keep your gems and nobody will be able to steal them. However, you won’t be able to sell or perform rituals with these gems either.
To save your gem in the Treasure Hall, you need to spend some Emeralds. Emeralds is the new resource that is produced by performing rituals. The number of Emeralds depend on the value of the gem that you get after the ritual is finished. The more valuable it is, the more Emeralds you receive.
Each time you save a gem in the Treasure Hall, you increase your Saved Rating. If the Rating is high enough, you get a Magical Sphere that contains Unique gems. If you save gems with more than 1 000 000 value, you get a bonus Sphere.
You can find all gained Unique Gems in Collection. By the way, when you add a Huge Gem your Treasure hall, it will be counted in Collection too.
Beginners, who have just installed the game and started it, will get access to Treasure Hall after they move to the third dungeon.