(as|) good as gold

used to describe a very well-behaved and obedient individual, especially a child

A lot of teenagers rebel, but not me—I was always good as gold.
She's been as good as gold all morning.
The children were as good as gold, waiting patiently at the entrance.

(of a thing) in a very good or desirable condition

The idiom "as good as gold" originally meant "genuine" or "authentic," with the allusion to gold as a precious metal. It was used to describe the authenticity or reliability of something, and it dates back to the late 1600s. Over time, the meaning shifted to primarily describe well-behaved or obedient behavior, especially when referring to children or animals.

The antique vase she bought turned out to be good as gold, a genuine piece from the 18th century.
Despite being mischievous at home, the children were as good as gold during the school field trip.
The mechanic assured me that the used car he sold me was good as gold, and it has been running perfectly.
The old watch may not look like much, but it's as good as gold, still keeping perfect time after all these years.
The puppy was as good gold, never causing any trouble and always following commands.

(of a person) having good health or positive outlook on life

The origin of the idiom "good as gold" is unclear. However, it likely stems from the association of gold with value, purity, and positivity. Gold is often used metaphorically to represent something valuable or precious, and in this case, it represents good health and a positive attitude. It is used to suggest that the person is in excellent physical condition or possesses a cheerful and optimistic attitude.

After recovering from the illness, he's feeling good as gold and ready to get back to his normal routine.
Despite facing challenges, she remains as good as gold and always has a smile on her face.