925 Sterling Silver is 92.5% pure silver mixed with 7.5% alloy as per market standards. The purest form of silver which can be used to make sturdy jewelry as 100% is very soft to make the same. So we mix 7.5% non-allergic alloy to make it hard enough for manufacturing. It never tarnishes or turns black. Can become pale or light yellow after a few years of use. Can lose its shine with daily use.