Your Insurance comapny should be able to provide a reputable jewelery appraiser. It's always a nice feeling to have it appraised for a higher $$ value than you paid, but the MOST important thing is that the description of the item be as accurate as possible (especially with gemstones). Most insurance operates on replacement value, so it doesn't really matter what the monetary value is listed at (that will only inflate your premium), the description will be what they base the replacement value on, if you are unfortunate enough to have a loss. Hope that helps. Here in NY we have a IGI (International Gemmological Institute), which is almost as good as GIA (California), but I don't know if they have offices in your area.
I'm sure you've heard the stories before, but NEVER let a jeweler out of your sight with your gemstone.