The most expensive color of glass to produce historically is cranberry red or gold ruby glass, which requires the actual addition of micro-quantities of pure dissolved gold chloride into the molten glass batch to achieve its rich, vibrant hue. Because gold is an exceptionally precious metal, mastering the precise thermal reduction chemistry needed to strike the ruby coloration makes authentic gold-infused antique glassware highly coveted by collectors and exceptionally expensive compared to standard glass colored with cheaper metallic oxides like iron or cobalt.