Mary knows no use for gold. It doesn’t protect, it doesn’t sanctify, and it doesn’t poison. You can bless it or shape it, but its utility only arises after you’ve processed it, not because of some arcane supernatural equation.
Mary loves wearing gold. She loves her wedding band and the simple locket John gave her on her birthday; the accompanying card was signed with his sloppy print and Dean's enthusiastic scribble. She loves its warm color and simple beauty.
Mary doesn't miss wearing silver and iron. She fills her locket with salt because it's a way to honor her parents.