There are many different types of opal. Some are very valuable and some are reasonably priced. There are also different types of cabs as well, such as "doublets, triplets", etc. Doublets are made by gluing a thinned layer of opal onto a cheaper base stone such as onyx. Triplets are made by sandwiching a thin layer of opal between two stones, one being a clear quartz piece on the top, allowing the colors of the opal to show and actually be magnified slightly.
Also, an important factor is the location the opal came from originally, as well as the color.
So, there are so many variables with opal that it will take a little time to determine what you have. A jeweler should be able to narrow it down for you.
Good luck! Let us know what you find out.