Manipulating index numbers
The best way is taking the index "ingredients" and rebuild it... assuming you manage the index data on a DB or on an excel file, that should not be too difficult

Otherwise, assuming you're index ingredients are averaged equally, then in a sense, the index is calculated as:

Index = (1 + Product1ChangeIn%)^(1/9) * (1 + Product2ChangeIn%)^(1/9) ...


So based on it, without coal, you're index should be:

Index / (1 + CoalChangeIn%)^(1/9)