Full Idea
If real essences are decided by microstructure, then what we call the element tin is not a natural kind, but a mixture of 21 different kinds, one for each isotope. There also exist two different allotropes of tin - white tin and grey tin.
Clarification
'Isotopes' vary in their numbers of neutrons; allotropes differ in their structure
Gist of Idea
Tin is not one natural kind, but appears to be 21, depending on isotope
Source
Alexander Bird (Philosophy of Science [1998], Ch.3)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.107
A Reaction
This example vividly brings out the difficulties of the Kripke-Putnam view. If natural kinds 'overlap', then there would be a very extensive overlap among the 21 isotopes of tin.