Mood rings were first seen as an extremely popular fad, in the late 1970s, and they resurface regularly!
The idea behind a mood ring is simple: Wear it on your finger and it will reflect the state of your emotions. The ring's stone should be dark blue if you're happy, and it supposedly turns black if you are anxious or stressed.
While mood rings cannot reflect your mood with any real scientific accuracy, they actually are indicators of your body's involuntary physical reaction to your emotional state.
The stone in a mood ring is either a hollow glass shell filled with thermotropic liquid crystals, or a clear glass stone sitting on top of a thin sheet of liquid crystals (see , for lots of details on liquid crystals). Work
These liquid crystal molecules are very sensitive; they change position or twist, according to changes in temperature. This change in molecular structure affects the wavelengths of light that are absorbed or reflected by the liquid crystals, resulting in an apparent change in the color of the stone.
For example, as the temperature increases, the liquid crystal molecules twist slightly in one direction. This twist causes the liquid crystal substance to absorb more of the red and green portions of the visible light, and reflect the blue part. This causes the stone to appear dark blue.
When the temperature decreases, the molecules begin to twist in the other direction and reflect a different portion of the spectrum.