The opal is one of the few gemstones to come from a mineraloid rather than a mineral. The name opal itself comes from the Sanskrit "upala", which means "precious stone". While found most plentiful in Australia, it has also been found in Eastern Europe and South America.
Opal comes in many different colors and transparencies. Some are translucent, while others are nearly entirely opaque. Its luster is just as varied in the cloudier form, as it ranges from vitreous to pearly. Opals can come in just about any color and almost always contain more than one color. This seemingly random distribution of color within a single stone goes hand in hand with the fact that opal possesses no crystalline structure.
One of the unique things about the opal is in the way it diffracts light. Disparate from other gems, Opal is so thick that light cannot pass through it and thus a "play of color" results. This is the phenomena of seeing the spectrum of colors as you look into the Opal, also called iridescence. Another factor that distinguishes this gem from others is that each has a more unique appearance than diamond or an emerald.
Opals are more often than not shaped into cabochon rather than faceted. Unlike more fragile gems, Opal does well with being worn often as it contains a high amount of water and benefits from more humid, warmer environments. It does rate a bit soft on the Mohs Scale, however, and so proper care should be taken in its setting and handling. Many superstitions surround the opal, but one of the most common is that the stone brings good luck because of the inclusion of bits of colors representing all the other gemstones.