New Jedi Academy
Glowing blue lightsaber crystal
The Bellum crystal produces a blue glow when near force users, the brightness increasing with the greater level of the force whether it is a number of people or just one powerful being.
When attuned to the wielder, the Bellum crystal gives a +2 bonus on Use the Force checks to Sense Force.
The crystal glows when in the presence of the force within 30 meters (20 squares). This allows anyone present with the crystal to know an increase or decrease in the general force levels of the area.