Crystal-mine key

The Crystal mine key is found during the Haunted Mine quest. Towards the end of the quest, you fight a poltergeist named Treus Dayth when trying to pick up an "innocent looking key". After beating him, you can pick up the key, which is the crystal mine key. Not only does it allow passage deeper into the mine where you can mine the crystal that makes a Salve amulet, it also allows full access to other doors in the mine. This makes it much easier to get to the bottom, since you can go straight through the front door and all the way down the stairs.

This key can be placed on the Steel key ring.