How do mood rings work?

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 How LCDs Work for lots of details on liquid crystals).

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.

The inside of the ring conducts heat from your finger to the liquid crystals in the "stone." The color green, which signifies "average" on the mood ring color scale, is calibrated to the surface temperature of a typical person, approximately 82 degrees Fahrenheit (28 degrees Celsius).

If your surface temperature varies far enough from the norm, then the liquid crystals in the stone alter enough to cause a change in the color reflected. And if you take a mood ring off, it will normally change to black unless the ambient temperature is very high.

When you are in a passionate mood, your skin is usually flushed. This is a physical reaction to an emotion, causing the capillaries to move closer to the surface of the skin and release heat. This brings about a slight change in the surface temperature of your body.

When you are nervous or stressed, your skin may feel clammy. This physical reaction to your emotional state causes the capillaries to move deeper into your skin, causing the surface temperature to drop.

Get it? : )

I believe there is a chemical that changes color due to body heat.