Gold fill (also called gold overlay or gold-filled) is made by using heat and pressure to apply a layer of karat gold to a base of less costly metal. This produces a surface with karat gold. The minimum layer of karat gold must equal at least 1/20 of the total weight of the item.
Gold fill chain lasts years or even decades longer than gold plated chain, because it is 50 to 100,000 times thicker than normal gold plating, and 17 to 25,000 times thicker than heavy gold electroplate (sometimes stamped 14kt HGP). See illustration for more info.