Opal is a
silica mineral, like
chalcedony, but it has a lot of
water in its structure and is more like a hardened
gel than a mineral with a
crystal structure of
molecules in rows. Though opals are found all over the world,
Australia is the biggest source (including finds like Big Ben, a stone of over 4,000
carats).
Gemstone opal is the precious kind mentioned in Webster 1913's definition, and it is divided into "white opal," "black opal," "fire opal," and sometimes "water opal." What makes all of these "precious" opal" is the fire, a type of color play with flashes of color coming from little spheres of silica suspended in water inside the stone. The more red/orange the fire, the more expensive the stone will likely be. (Though blue/green fire has its own appeal.)
- White opal is the most common -- a milky white background with flashes of other colors. This is what most people think of when they think of opal.
- Black opal refers to opal with a black background (some dark-but-not-black stones are called "semiblack" opals). This is the rarest and most expensive type of opal.
- Fire opal doesn't really look like other opals -- it is more transparent and has little to no fire (the name comes from the color of the whole stone, which is red-orange). Fire opals could usually pass for a reddish topaz, and they are usually cut faceted, unlike opals with fire which don't need anything to make them look showy.
- Water opal is rare and little-known; it is a completely colorless opal with fire.
Opal is sometimes found in thin
sheets attached to some other type of
rock, narrow seams running through them, or just flecks of opal through ironstone. It can be sold this way, attached to or scattered through the "motherstone," and this is called "
boulder opal." Often these are just meant as decorative mineral specimens, but boulder opal can also be made into
jewelry.
Because of its softness and the thin layers available, opal is often made into doublets or triplets, where a layer of opal is either a) covered with some transparent material to protect its surface; b) layered on top of some other material to make it look darker and the fire show up more; or c) layered with several layers of opal to give the appearance of a thicker stone. The glue in these doublets and triplets has been known to sometimes change color over time. Another occasional problem is that the stone dries out, losing the water in its structure; some experts recommend soaking the stone in water briefly every so often to prevent this.
Thanks to Siobhan for additional info.