Cut and polished opals come from different places in Australia. Here you will find most commonly three types:

1. Black opals from Lightning Ridge, NSW.
2. White/Milky or Crystal opals from Coober Pedy, SA.
3. Boulder opals from Winton, QLD.

Some miners will cut and polish their rough, others will sell it to jewelers and cutters.
Cut and polished opals are more expensive than rough, since you can see exactly what you are paying for.
The value of an opal stone is a highly individual opinion, but most opal sellers will price their opals based on the background (Black > White > Crystal), and amount and brilliance of the colour. Some rare colours are more expensive (Red > Orange > Green > Blue).