Full Idea
The complex idea of gold includes its being something which has a real essence whose detailed constitution is unknown to us, except for the fact that such qualities as malleability depend upon it.
Gist of Idea
Gold has a real essence, unknown to us, which produces its properties
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 3.10)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.345
A Reaction
This is precisely the view of modern scientific essentialism. The underlying idea I take to be the conception of essence as the thing which explains the properties.