Answer:
Answering from personal experience, if you are a well-seasoned wire artist, use 24g sq soft wire with 21g 1/2 round for wraps - and you can make either a lovely pair or earrings or a double cab pendant! I suggest working 'off the stone' which means create your frame around an item that is the same size, (like another cab) and make the frame just a tiny bit larger than you would on a harder stone; pull the wires on the top first, then remove the substitute cab and insert the opal, finish by making the pulls needed on the back, locking the opal into the frame. (I wouldn't use them in a high traffic item such as a ring or bracelet though.)
Answer contributed by Dale "Cougar" Armstrong