Full Idea
It is very true that it is part of the complex idea of gold that it is a thing which has a real essence, the constitution of which is not otherwise known to us in detail.
Gist of Idea
Part of our idea of gold is its real essence, which is not known to us in detail
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 6.6.345), quoted by Nicholas Jolley - Leibniz and Locke on Essences p.201
Book Reference
'Leibniz: Critical and Interpretive Essays', ed/tr. Hooker,Michael [Manchester 1982], p.201
A Reaction
See also Idea 12807. This is the clearest possible statement of Leibniz's clear-cut scientific essentialism, here presented in opposition to Locke (thought I take the latter to be only bothered by our inability to know the hidden constitution).
Related Idea
Idea 12807 The word 'gold' means a hidden constitution known to experts, and not just its appearances [Leibniz]