A mood ring is a ring which contains a thermochromic element, such as liquid crystal. The ring changes color in response to the body temperature of its wearer. The color is said, by some proponents, to indicate the emotional state of the wearer.

The mood ring was invented in the late 1960s by Marvin Wernick when he accompanied a doctor friend to a nearby emergency. When the doctor pulled out a strip of thermotropic material to gauge the child's temperature by applying the strip directly to his forehead, jewelry designer Wernick knew he had the makings of a winning item. Wernick encapsulated ovals of the material within clear glass cameos and glass domes set in brushed gold and silver ring settings.

His signature "hang-tag" contained claims for the ring's properties. A mood ring is a specialized liquid crystal thermometer, wearable on the finger. The ring is typically ornamented with a faux gemstone (usually made of quartz or glass) which is either a clear capsule filled with thermochromic liquid crystal, or has a thin sheet of liquid crystal sealed underneath.

Changes in temperature cause the crystal to reflect different wavelengths of light which changes the color of the stone. The liquid crystal used in mood rings is usually set up to display a "neutral" color at the average human skin temperature, which is approximately 91°F (33°C).

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