The necklace sounds lovely...pics? I love opals

You might be best to take it into a jewelry shop and ensure they know proper handling of opals. They may need to remove the stone from its setting in order to clean the silver.

Another (but more expensive) option would be having the stone re-set in white gold rather than silver so that the tarnish problem is eliminated entirely. Something to consider for the future, at least, if it's of heirloom quality.