Full Idea
When Archimedes asserted that something was gold, he was not just saying that it had the superficial characteristics of gold; he was saying that it had the same general hidden structure (the same 'essence', so to speak) as any normal piece of local gold.
Gist of Idea
Archimedes meant by 'gold' the hidden structure or essence of the stuff
Source
Hilary Putnam (The Meaning of 'Meaning' [1975], p.235)
Book Reference
Putnam,Hilary: 'Mind Language and Reality: Papers vol 2' [CUP 1975], p.235
A Reaction
This is one of the key announcements of the new scientific essentialism, and seems to me to be totally correct. Obviously Archimedes could say 'this is really gold, even if it no way appears to be gold'.