Opal is noncrystalline hydrous silicon dioxide. In other words it has the same chemical composition as quartz, but contains 1-2% water and it is not crystallized. It is composed of alignments of tiny spheres which form a compact, three-dimensional network. It is the play of light off of these tiny spheres that often gives opal its unique internal iridescence which is called "opalescence"!
If that opalescence is present, the opal is considered precious opal or sometimes noble opal. Precious opal is generally the most sought after and expensive opal. It is what most people think of when they speak of opal. I
If the opal doesn't display this internal iridescence, if it is just semi-opaque to translucent it is called common opal or potch opal. Common opal sometimes has a somewhat attractive, porcelain-like appearance and is suitable for beads.
In some cases common opal is very translucent to transparent with a bright red, orange or yellow color. This makes very beautiful gemstones and is called "fire opal". That term can be a little confusing because the "opalescence" in an opal is often referred to as its fire. But fire opal doesn't necessarily have any "fire"! Fire opal comes mainly from Mexico. They can be cut into cabochons or even faceted. In some specimens there will be some internal iridescence, therefore qualifying it as precious opal.
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