Special kind of of glass used in making various smoking utensils. Like the name implies, it changes color as you smoke out of the utensil.
Check out my cool color changing glass spoon!
CCG (as it's known) is made by small pieces of metal (usually silver and/or gold) that are held in the flame of the torch until vaporized (often called 'fuming'). The glass is rotated in this vapor stream until a fine layer of the vaporized metal is visible on the surface of the glass.
After this, clear glass is applied (often in 'dots' and 'lines') to 'encase' the vaporized metal in between layers of glass. There are many techniques and methods of applying the fumes and clear glass making a myriad of different colors, and endless types of patterns that one can come up with. The colors produced with fumed metals are best when they are translucent, vibrant, and reflective. As the pipe darkens with use, the colors (many barely visible) become darker and much more conspicuous, thus causing the appearance of much richer 'color change' (the glass itself doesn't actually change colors though).
Color changing glass is used in a variety of products such as glass vases, oil lamp bases, perfume bottles, containers, smoking accessories, and so forth.