Low grade alloy silver was mainly composed of 60%silver or more and 40%copper or less. The greater the copper content, the darker, harder, and more easily oxidized the metal becomes. Coin silver with 80% silver and 20% copper is harder and darker than, say, sterling silver with 92.5% silver and 7.5% copper (Note: by darker I assume that the metals in question have been cleaned, which, BTW raises the value in this case and makes it look shiny

).