This is a simple design; the type of optical gain medium used usually defines the type of laser. It can be a crystal, examples are ruby and a garnet crystal made of yttrium and aluminum with the rare earth metal mixed in. Gases can be used for laser by using helium, nitrogen, carbon dioxide, neon or others. Large, powerful lasers are usually gas lasers. A free-electron laser uses a beam of electrons and can be tuned to emit different colors. Finally, the smallest lasers use semiconductor diodes to produce the light. These are the most numerous kind, used in electronics.