Opals, while rare in real life, are relatively easy to come by in your kingdom in Castleville Legends. Not only does Yvette's House produce two of them for you every four hours (assuming you keep adding royal coin that often), but you can also find them by adventuring in certain of the Places of Legend. And should you still run short, you can occasionally find them being sold, singly or in bulk, in the Sea Trading marketplace.
For a discussion of the best items to make and sell with opals (suited for lower levels), scroll to the bottom of the Yvette's House page.
12 Royal Coin
Maximum sale price in sea trading:
Myra's purchase price:
290 royal coin, unknown XP
Recipes using this item:
(Click on an item to learn more about it)
Sea Gem Cup