Rubs are pieces of opal that have been taken to the lapidary wheel to begin exposing any gem colour present. Rubs may also have been partly shaped, or preformed. They have not been given their final shape and polish.
Rubs are often more expensive than rough opal, because there is usually less risk involved than when purchasing rough opal: when buying a rub, the purchaser can see better how much opal colour is present and of what quality. Some people prefer to buy rough opal because it is exciting and may yield high dividends; others prefer to buy rubs because the risk is lower.