Gold gives stained glass its red color when added during manufacture.
Small quantities of gold compounds can produce ruby-red glass, although the exact color depends on the glass mixture, firing conditions, and the way the gold particles develop within the glass. This is an example of color created in the glass itself rather than painted onto its surface.
Gold ruby glass became especially important in later European glassmaking because it offered a rich red that was difficult to achieve consistently with older recipes. The material is expensive, so recipes use only very small amounts. The final shade can range from pinkish tones to deep ruby.
This process should not be confused with silver stain, which is applied to the surface of already made glass and commonly produces yellow or amber effects after firing. Cobalt, meanwhile, is strongly associated with blue glass, not gold ruby red.