The manufacture of crystals.
Me and My Mood mood rings change color in response to temperature because they contain thermochromic liquid crystals.
There are several natural and synthetic liquid crystals that change color according to temperature, so the exact composition of a mood crystal depends on its manufacturer, but most rings contain crystals made from organic polymers. The most common polymer is based on cholesterol.
As the ring becomes warmer, more energy is available to the crystals. The molecules absorb the energy and essentially twist, altering the way light passes through them.
Natural crystals without flaws being far too rare, the industry manufactures synthetic quartz by crystallizing silica in the same crystalline system as nature.
In the nature this phenomenon is called hydrothermal (= underground circulation of hot water, loaded with minerals). The waters circulating in deep fractures in the continental crust are at high temperature and high pressure, and their Ph is sometimes quite high.
These waters can dissolve much silica at great depth. If during their course they rise (drop in temperature and pressure), then the silica crystallizes on the edges of the fracture (which serve as seeds), and very beautiful crystals of quartz develop.
Crystals can also be made chimically.
Quartz is soluble at high temperature and pressure, and at basic pH. Natural quartz is dissolved in water at very high temperature and very high pressure (each manufacturer has its secret). Over time (several weeks), these quartz slivers that serve as seed of crystallization grow and become beautiful quartz crystals.