What's an alternative for "hidden gem"?
Solution 1:
I don't know if there is an idiom that covers everything, but you may find use in these:
few and far between (rare and difficult to find)
very few; few and widely scattered. Get some gasoline now. Service stations on this highway are few and far between.
Some people think that good movies are few and far between.
scarce
- Insufficient to meet a demand or requirement; short in supply: Fresh vegetables were scarce during the drought.
- Hard to find; absent or rare: Steel pennies are scarce now except in coin shops.
once in a blue moon (rare event)
Rarely, once in a very long time, as in We only see our daughter once in a blue moon. This term is something of a misnomer, because an actual blue moon-that is, the appearance of a second full moon in the same calendar month-occurs every 32 months or so. Further, the moon can appear blue in color at any time, depending on weather conditions. [Early 1800s]
hole in one (rare achievement)
A perfect achievement, as in Tim scored a hole in one on that test. The term alludes to a perfect stroke in golf, where one drives the ball from the tee into the hole with a single stroke. [c. 1900]
Solution 2:
Admittedly this is not precisely an answer to the question as asked, but be not too hasty to reject hidden gem as unsuitable for formal discourse.
Formal discourse does not eschew idiom provided that the idiom in question is neither conspicuously vulgar nor excessively specific to a single region or social grouping; it aims at a certain universality. In this, the use of formal English resembles the use of Latin, rather than vernacular languages, in pre-modern Europe, for academic as opposed to literary texts (which more commonly were in vernacular languages): to write in Latin was to write for an international community of scholars whose vernacular mother tongues might vary from Icelandic to Portuguese to Low Dutch.
Hidden gem qualifies as a reasonably universal idiom, found at comparable frequencies in both American and British English. Its currency is likely at least partially owed to the following lines in Thomas Gray’s “Elegy Written in a Country Churchyard”:
Full many a gem of purest ray serene
The dark unfathom’d caves of ocean bear:
Full many a flower is born to blush unseen,
And waste its sweetness on the desert air.
Needham Bryan Cobb in 1879 published a poetical “Reply” to these lines specifically, subtitled “the unseen rose—the hidden gem.”