EMBRACE YOUR IMPERFECTIONS
YAG crystals are man-made garnets. They can be used as gemstones for jewelry, but their main use today is in the laser industry. Interestingly enough, perfect YAG crystals are useless for lasing. It is literally their imperfections that make them great. I’m telling you, there’s a t‑shirt idea in there somewhere.
CORRECTION: WHAT ARE YAG LASERS? PLURAL.
As mentioned earlier, yttrium aluminum garnets (YAG) must contain some impurities to be useful as a lasing material. These impurities, or dopants, come in a wide variety, including erbium (Er), neodymium (Nd), ytterbium (Yb), and many others.
Because so many different dopants can be used, it is a bit ambiguous to speak of a YAG laser in the singular form. That being said, neodymium-doped YAG lasers (Nd:YAG) are the most common and you can usually assume this is what people are referring to when they simply say “a YAG laser”.