They're for remodeling. Not every furniture item can use them though. He'll only mention them if you have them in your pockets. They also sell for quite a bit.
If you use the gem ore types, you get a kind of crystal effect in the colour of the ore and gold will just turn the furniture gold (actually not sure if Cyrus will conjure a piece of furniture out of three gold ores or if you also need to bring an item for him to turn gold). I figure the silver works like gold, but I've actually not read about that.