Looking for an expression of extreme surprise.
I've got shock, amazement, aghast and the rest of the the thesaurus, but I'm really seeking something more idiomatic.
Ideally the phrase/idiom would exude positivity, such as finding a treasure.
You might use:
Reverso says of beyond belief:
You use beyond belief to emphasize that something is true to a very great degree or that it happened to a very great degree.
Here's an example usage from the book Secrets from the Operating Room: My Experiences, Observations, and ...:
I was surprised beyond belief and was excited.
not believe one's (own) eyes
: to think that something you see is not likely to be real She looked so different in a uniform, I couldn't believe my eyes. Usage notes: sometimes scarcely or hardly are used instead of not: When he found the ring in the grass, he could scarcely believe his eyes Cambridge Dictionary of American Idioms
knock/throw for a loop; knock down or over with a feather; knock sideways.
Overcome with surprise or astonishment, as in Jane was knocked sideways when she found out she won. The first two of these hyperbolic colloquial usages, dating from the first half of the 1900s, allude to the comic-strip image of a person pushed hard enough to roll over in the shape of a loop. The third hyperbolic term, often put as You could have knocked me down with a feather, intimating that something so light as a feather could knock one down, dates from the early 1800s; the fourth was first recorded in 1925. The American Heritage® Dictionary of Idioms.
- Fig. to overwhelm someone with beauty or grandeur; to surprise or astound someone. The magnificent painting took my breath away. Ann looked so beautiful that she took.
The Free Dictionary
The first phrase that comes to mind is "shocked delight". I've also seen "to his amazement and delight".