The Umoona Opal Mine and Museum on Hutchison Street is the right first stop in Coober Pedy. The combined attraction includes a walk-through opal mine, displays on the geology and history of mining in the area, a small Aboriginal cultural exhibit, and a furnished underground house showing how locals live. Guided tours run several times a day in season.
The on-site shop sells opals from rough stones up to finished jewellery, and the staff are happy to explain what you are looking at — useful before you buy anywhere in town. Allow at least 90 minutes; longer if you want to watch the introductory film.