Regardless of type, any grinding mill that uses silica gravel as the crushing medium is called a pebble mill. These mills are mainly used to process ores whose products are susceptible to iron contamination, such as gold and silver ores. This is because the iron filings produced by the grinding of iron balls and liners are harmful. In such cases, silica gravel liners are used, or the mill’s inner walls are lined with silica square bars embedded in cement.