Thermotropic liquid crystals makes a mood ring change colors.
They’re little crystals that change molecular states when the temperature changes. When their molecular states change, the crystals’ ability to absorb light also changes, meaning the color you see will change, too.
The ring picks up body heat from your finger, and transfers it to the crystals in the ring.
The crystals are calibrated to reflect the color green at a surface temperature of about 82 °F. When the surface temperature gets cooler, they go darker and then gray/black. When they heat up, the color changes to blue hues.
Does the ring actually show your emotions? Well, not exactly. It only reflects the small differences in surface temperature on your hand.
However, the circulation in your hands will generally tend to increase when you’re relaxed, increasing the surface skin temperature, and will sometimes decrease with tension, lowering the surface skin temperature.
Based on that change, the mood ring instructions tell you to read your ring like this: Dark blue romantic, passionate, happy; blue completely soothed; blue/green relatively calm; green normal; brown/ amber slightly uptight; gray uptight; black completely wound-up and stressed.