Once you’ve started exploring the Raccoon City Police Department you will begin to see that there are number of places that you cannot go because of locked doors. Some of these locked doors are going to have symbols on them. There are heart, diamond, club, and spade doors and they’ll all require a unique key to get into.
The Diamond Key will be found in the Morgue in B1. Once you’ve progressed past the Medallion Puzzle and into the Underground Facility, past your first encounter with Birkin and into the Parking Garage you will be able to go into the Morgue and get the Diamond Key which can then be used on a number of doors in the Police Department. Any door with the Diamond shape on it can be opened with the Diamond Key.
Diamond Key Map
When you head into the Morgue for the first time you’ll want to be prepared. There are numerous zombies in there that will try to attack you in the process of getting the key. You can go in and out without getting hit with a little practice. As soon as you head in the door go straight and around the tables/sinks in the middle of the room. Pull out the table in the wall second to the left and the key will be next to the corpse’s left shoulder.