Full Idea
There isn't any XYZ, and there couldn't be any, and so we don't have to worry about it.
Gist of Idea
XYZ (Twin Earth 'water') is an impossibility
Source
Jerry A. Fodor (The Elm and the Expert [1993], §2.I)
Book Reference
Fodor,Jerry A.: 'The Elm and the Expert' [MIT 1995], p.29
A Reaction
Jadeite and Nephrite are real enough, which are virtually indistinguishable variants of jade. You just need Twin Jewellers instead of Twin Earths. We could build them, and employ twins to work there.