Gems
From KeenWiki
Gems are items used to open doors in Keen 4, Keen 5, and Keen 6. They come in the colours red, blue, yellow and green. In order to open a door, the gem is placed in a holder of the corresponding colour. The door then moves backwards, becoming part of the background and allowing Keen to pass.
A corresponding system in Keen 1-3 is the Keycards.
