Mood Ring

The idea behind a mood ring was simple: You wore it on your finger and the colour of the stone would reflect the state of your emotions. And in 1975 it only cost $19.95 to figure out how you were feeling. 

There were many incarnations of the personal mood-detector. There was a big flat masculine-looking ring, a watch that changed colour (the moodwatcher), pendants and even nail polish.

Martin Landau and Barbara Bain (from Space 1999) showed up on a TV show wearing matching gold ones. They showed them to the cameras and the studio audience applauded. They later divorced.

By 1977 the Mood Ring had all but disappeared, although it enjoyed something of a revival in the 90s and up until today.

How did they 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.

These liquid crystal molecules are very sensitive; they change position 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 colour 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 . . . which explains why they were totally useless outdoors in winter when they would just turn grey and stay like that.