Full Idea
Emeralds cannot all turn blue in 2050 (as Nelson Goodman envisaged), because to do so they would have to have an extrinsically variable nature.
Clarification
'Extrinsically' here means imposed from the outside
Gist of Idea
Emeralds are naturally green, and only an external force could turn them blue
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.7)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.137
A Reaction
I was never very impressed by the 'grue' problem, probably for this reason, but also because Goodman probably thought predicates and properties are the same thing, which they aren't (Idea 5457).
Related Idea
Idea 5457 Predicates assert properties, values, denials, relations, conventions, existence and fabrications [Ellis, by PG]