The oxide minerals all contain one or more oxide anions (O-2). The oxide minerals include species that are hydroxy-oxides. The hydroxide minerals (those with one or more OH-) are usually considered together with the oxides. Many sulfide minerals are not stable in Earth-surface conditions. In the presence of oxygen and moisture, sulfide minerals tend to tarnish or alter to oxides and hydroxy-oxides. All except the most inert elements (such as the platinum-group elements and gold and noble gases) readily form oxides. Gold oxide forms only under special conditions.