I have just been reading an article in Esoterica Volume IX (2007) Of Ether and Colloidal Gold: The Making of a Philosophers’ Stone by Hereward Tilton .
In this Tilton explores a hypothesis that the Philosophers' Stone prepared by some alchemists might have been a form of colloidal gold. In the early part of the article he focusses on some writings of Heinrich Khunrath, and suggests that one could read these as dealing with the preparation of colloidal gold, which can appear in a red or purple form (as with the pigment Purple of Cassius). He further explores the possibility that alchemists could have created the organic solvent diethyl ether, long before it is recorded in the history of science. Using this ether, a form of colloidal gold could have been made which might appear to have the properties of the Philosophers' Stone - a dark red waxy substance that tinges metals. This colloidal form of gold would rapidly turn to a conventional metallic form on being exposed to high temperatures, through being projected or thrown onto molten metal, then it might actually coat the metal with a thin film of metallic gold. Tilton does not mention if he has repeated this in a practical experiment. For all one knows the gold might immediately dissolve and sink in the molten metal and one would probably have to do the experiment to find out what happens with the different metals and quantities involved.
I have entertained this idea in the past, but rejected it as I felt such a thin surface film would deceive no one, even for a minute. I thought that even a fingernail would readily scratch below the layer of gold, especially if it was on a soft metal such as lead. Conventional gilding used gold leaf or gold-mercury amalgam, and from the earliest times ordinary people were well aware of gilding. I cannot believe that anyone in the 17th century would be deceived for long by a thin surface film of gold.
That being said, it is an interesting article, which explores various other themes.