Water entered into oxides when the oxides were indented with diamond in the presence of water or water vapor. Similar water entry is expected during silica glass polishing because of the high stress concentration on the glass surface in contact with small abrasive particles. Both chemical and mechanical properties of silica glasses change with water content. Polishing rate of silica in oil-based slurry with diamond abrasive increased with water content in the silica. Since oil has no chemical reactivity with silica, mechanical property degradation with water incorporation must be responsible for the fast polishing rate of the water-containing samples. A plausible mechanism of the polishing of silica is, therefore, water entry into silica under stress and mechanical removal of the altered layer. In the presence of water this process repeats itself.