The dark crystal is one of the four crystals hidden throughout the environment as of Alpha 0.0.82. It is also known as the Fear crystal. It is a light purple in color, and glows with a deep purple light.
The dark crystal is located deep within the mines , the furthest room in the left corner behind the rock wall.
Touching the CrystalEdit
Nearing the crystal, it's labeled as Crystal (Unknown) until you pick it up. Once you do, the Fear effect is applied to your vision while holding (right-click default). After dropping the crystal, it is now labeled as Dark Crystal (Fear) and the Fear effect fades.