From Ars Magica
On the left is an empty keystone; on the right is a keystone with a combination of runes that will allow the user to interact with a specific network on a Gateway system, avoid the effects of a matching Astral Barrier, or trigger the effects of a matching Seer Stone. A Keystone has 5 slots to place any combination of Runes in it. There are 32,768 possible combinations of runes that can be placed in a Keystone.
Used in Gateways, Seer Stones, Astral Barriers, and Keystone Doors.
The Keystone Chest is a lockable invincible chest.
You lock it placing runes in any pattern, you unlock it with a second keystone of the same combination.
You can't use a hopper to put things in them, however you can use a hopper to remove items and the keystone.