Gold is not found free in tissues, and does not generally need to be demonstrated. A possible exception would be residual gold from medical treatment with radioactive gold. In this case, its presence would be known from the patient's history, and would not cause a problem with identification. Finely precipitated gold is purple. Larger deposits are likely to be black.
Gold's commonest use in histotechnology is as a toner to change the colour of a silver impregnation from brown to black. It is also used, in conjunction with mercuric chloride, as an impregnating metal itself in Cajal's gold sublimate method for astrocytes. Gold is used in histology laboratories as the sodium salt of chloroauric acid, or as chloroauric acid itself. Older terminology refers to the sodium salt as yellow gold chloride or sodium gold chloride, while the acid may be called brown gold chloride. It should be noted that the compounds are not simple chlorides, and that the gold-chlorine complex is an anion. It is also referred to as hydrogen tetrachloroaurate.