There are very few jewellers that know the value of
any stone. They are merchants. Go to a good G.I.A. certified appraiser for the value of a gemstone. Opals can range in price from several dollars a carat to fifteen/twenty thousand dollars a carat easy. Opals can be white, black, or crystal. Overall colour and intensity being roughly the same the blacks are most expensive due to their beauty and rarity. There are no strict guidelines for valuing an opal. Even experts will vary somewhat in their appraisal due to personal preference as to colour and pattern. It is difficult to value an opal due to each stone, for the most part, has its own personality, especially when you get into the better gem grades. Opals are generally rated on a one to five scale, five being brilliant. One are junk, twos are considered commercial grade. Most of what you see in a store, unless it is manmade, is the junk. There is not a good market for fine opals in the U.S. as most people have not seen any and consider it a cheap stone, even though it can sell for more per carat than a D flawless diamond.
Any jeweller or gem smith. The value is based on the polish, size, clarity, common market, and rarity of the stone.