I think that gem quality opal is amongst the most spectacular of gemstones, forget your boring diamonds (I say that with tongue in cheek) but the play of colour on a black opal for example can be mesmerising. The finest quality opals can fetch prices that are up there with the most expensive diamonds and emeralds.
Opal is quite a soft gemstone and has a hardness of 5.5 - 6 on the Mohs scale, because of this it is best suited to brooches or earrings or pendants where it is not coming into contact with daily wear and tear. When it is set in a ring it is generally bezel set to protect the stone in a metal surround so its not as exposed as it would be in a claw setting.
An opal is termed "precious opal" when it exhibits flashes of iridescent colours when its viewed from different angles, when the stone is moved or when the light source is changed. Its very rare and expensive and found in a limited number of locations worldwide. The most precious is generally mined in Australia such as this Lightning Rigdge Black Opal below.