Yup. A whole lot of chemistry process is just X dissolves in Y but not in Z, and using that in order to separate and purify. In this case metal oxides dissolv

Yup. A whole lot of chemistry process is just X dissolves in Y but not in Z, and using that in order to separate and purify. In this case metal oxides dissolve in glass (sand, which is a silicon oxide, mostly) but gold doesn't A) oxidize under reasonable conditions or B) dissolve in the glass. Sand or glass waste is melted, the not gold dissolves into the molten glass.