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I'm looking for a good idiom to express the idea of a clue or a key which if found can unravel or solve a mystery or greater problem. A lynchpin to a mystery or diagnosis with implications of it being hidden or obscured

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    Welcome to EL&U. Could you tell us what idioms you've considered and rejected, and why you've rejected them? Could you tell us what research you've done? – rajah9 Sep 8 '16 at 17:46
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    Rosetta Stone is often used figuratively. But key is a far more common metaphor for this context anyway. Note that figurative lynchpin means a person or thing *vital to an enterprise or organization*, which isn't at all the same thing as "key which unlocks a mystery". – FumbleFingers Sep 8 '16 at 17:56
  • @FumbleFingers: +1 for the "key of the mistery". – Graffito Sep 8 '16 at 21:25
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Here are some options with the idiomatic expressions in bold:

Once we found out that John Doe's fingerprints were on the gun, we knew we had the last piece to the puzzle. Today, he was found guilty.

John Doe's fingerprints were on the gun, and that did the trick. Today, he was found guilty.

John Doe's fingerprints were on the gun, and that sealed the deal. Today, he was found guilty.

John Doe's fingerprints were on the gun, and that unraveled the knot. Today, he was found guilty.

This one is clunky with the gun example I've been using, so I'll change it to knife.

John Doe's fingerprints were on the knife, and that was our smoking gun. Today, he was found guilty.

  • Are you actually quoting from some site? Or are these your own examples? :) – NVZ Sep 8 '16 at 18:08
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    This is me being pedantic, but only because I'm trying to understand what you're suggesting, not because this is personal. If by citing references you mean providing external links for the user's benefit such that he/she can find out more information about the topics I presented, I'm fine with that. But if you're arbitrarily adding links to suggest/guess where I got the information from (my own knowledge of the English language), that doesn't make sense. – arbitrarystringofletters Sep 8 '16 at 18:27
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    +1 For further reading, and for proof that these are not made-up expressions by you. :) – NVZ Sep 8 '16 at 18:28
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    No problem, just wanted to clarify. Thanks for the additions. – arbitrarystringofletters Sep 8 '16 at 18:29
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    I don't think the references add much, particularly since the examples aren't all that apt. The missing piece of the puzzle and a smoking gun aren't hidden at all: the former's shape per force is obvious; the latter's signal is likewise. Things that do the trick or seal the deal needn't be hidden and they needn't solve a mystery. If I offer another $100K to seal the deal, is the extra money not an obvious and unmysterious reason for the conclusion of the transaction? it's time and patience that unravel a knot. Which you do to solve a problem, not necessarily a mystery. – deadrat Sep 8 '16 at 19:07
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You're probably looking for a missing link (excuse the pun)!

M-W:

missing link noun

something that you do not have and that you need to complete a series or to solve a problem

Police are hopeful that the new evidence will provide the missing links needed to solve the crime.

scientists searching for the missing link

  • Aside from the unfortunate collision with paleontology of primates, a missing link is gone, not hidden. – deadrat Sep 8 '16 at 18:53

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