The phrase "a true gem of" is correct and usable in written English.
It can be used to describe something that is exceptionally valuable or beautiful, often in a metaphorical sense.
Example: "This quaint little café is a true gem of the neighborhood, offering the best coffee and pastries."
Alternatives: "a hidden treasure of" or "a remarkable example of".
Half an hour later, a true gem of the Dutch school left the Hals far behind.
But for all of the cliches and the tendency to fall back on lazy tropes, there is a true gem of a game in here.
She was a true gem of a mom and a gem of a person.
Scotland is famous for its rugged highlands, but a true gem of this country is the largest island of the Inner Hebrides, known as the Isle of Skye.
After staring at the front of the card for a few minutes as if I'd just found a Honus Wagner card and a true gem of baseball history, I yelled to my wife to come inspect this piece of Smithsonian history like a proud father.
"'The X-Files' was not only a seminal show for both the studio and the network, it was a worldwide phenomenon that shaped pop culture -- yet remained a true gem for the legions of fans who embraced it from the beginning.
The food trucks, the architecture, the white beach, the lack of ugliness make this town a true gem for the eyes.
And the one play excerpt is a true gem: Uta Hagen's ferocious emasculation of Arthur Hill in the original "Who's Afraid of Virginia Woolf?" Her condescending cackle will make you wince.
Damian Thomas of RPGFan said that the album was "a true gem" and said that despite his dislike of piano arrangements, he "truly appreciated" the album.
It also summoned Souhad Rafey, curator of exhibits at The American Academy of Arts and Letters, who was notably impressed with the artist's contemporary take on an ancient practice: "Each piece is an elegant fetish of sorts, a true gem in every way," she said.
Tom Day of MusicOMH wrote that the song "Before I Forget" is a "true gem and grinds out with a level of devastation that will make you green with envy if you weren't at these shows", and that drummer Joey Jordison took "centre stage" throughout the performance.
