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.”