Gatestone Geode

wonderous item, varies
Over the course of an hour, a Gatestone Geode can be configured and thrown on the ground where it then shatters. A new demiplane (called a Geode Vault) is created, with properties according to the rarity of the geode used. In the space the geode was used, a portal appears. The portal takes up a 5x5 or 10x10 foot space (chosen when the geode is consumed) and has a front and a back. Travel through the portal is only possible by moving through its front side. The portal can be opened or closed by the person who used the geode as an action, and while closed it is undetectable through nonmagical means. The vault also has a sigil unique to it, which is learned by the user of the geode upon creation. Certain other gatestones require the one useing them to know the sigil of a target vault. The sigil can also be learned by divination magic such as legend lore, assuming it originates from inside the vault.

Small Gatestone Geode
wonderous item, uncommon, 450 gp
The Geode Vault created by a small Gatestone Geode can be up to a 10 foot cube. The interior is always at a comfortable temperature.