Its color changes when used, depending on the Deeplings World Change stage.
When the Deeplings World Change is at stage 1, the color of the gem will be red, and a message will appear :
The power of the deep weakens. The heart emits warmth.
When stage 2 or 3 is active the color of the gem will turn blue, and a message will appear:
The power of the deep is still strong. The colour of the heart stays cool.
If you want it to go back to red you have to wait for stage 1.
Souce : tibiafandom