good as gold

In the sense of high and enduring value, with literal origins in trade (currency exchange and barter), where any given currency or commodity may be worth as much as gold or hyperbolically described as such.

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good as gold (not comparable)

1. (of an asset, especially New Zealand) Of high and enduring value.

   Synonym: goldlike

2. (especially UK, of a person, simile) Very well-behaved (especially of a child); very good-hearted (especially of an adult).
   • 1922 February, James Joyce, “[Episode 13]”, in Ulysses, Paris: Shakespeare and Company, […], →OCLC:

     But to be sure baby was as good as gold, a perfect little dote in his new fancy bib.