Sometime in 1987 or early 1988, I bought several pieces of pottery that had boulder opal and sterling attached. They were created by an Australian artist Georgina Elms, who did win an Australian regional/national pottery award a few years later. A jewelry store called Quilpie Opals, which mostly sold their own boulder opal jewelry and cabs, had about 10 of them available.
http://www.cookshill.com/exhibitions/2010/georgina-elms
I got 3 of them for a total of about $1100, and have enjoyed them for many years since.
Now I plan to donate one of them to a local charity auction. I am not concerned with an exact appraisal, but they want me to indicate a starting bid, and estimate a fair market value for the auction catalog.
Alternately I can sell to a more educated crowd and make a cash donation to the charity.