The mood ring uses thermotropic liquid crystals inside a glass dome or a strip of liquid crystals beneath a glass dome to measure the temperature.
Black - A black mood ring says that the person wearing it is feeling a great deal of tension, pressure and stress.
Amber - The person with an amber mood ring is feeling somewhat anxious.
Blue - A blue mood ring means the person wearing it is peaceful and calm.
For example, on a hot, muggy day the mood ring is far more likely to show blue than black.