Full Idea
For Locke, if we found out how to make some stuff which has the same nominal definition as gold, then we have found out how to make a new kind of gold.
Gist of Idea
If essence is 'nominal', artificial gold (with its surface features) would qualify as 'gold'
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by Antony Eagle - Locke on Essences and Kinds V
A Reaction
Unfair to Locke. He could see no way to get below the surface; we can do that. Obviously we will treat as gold any substance which we are utterly unable to distinguish from gold. Maybe we are doing that right now.