Full Idea
Part of what it means to be a natural kind is that they are defined by a real essence, a constitution that marks them out as the substance they are (as water is essentially H2O, and gold essentially has atomic number 79).
Gist of Idea
Natural kinds are defined by their real essence, as in gold having atomic number 79
Source
Mark Rowlands (Externalism [2003], Ch.6)
Book Reference
Rowlands,Mark: 'Externalism' [Acumen 2003], p.100
A Reaction
A 'real essence' would be the opposite of a 'conventional essence', which is just a human way of seeing things.