The NYRB Poets series has a volume devoted to Li Shangyin (c. 813–858) contaiing the work of three translators. All of them have a go at his most famous poem, 'Brocade Zither' (or 'The Opulent Zither' or 'The Patterned Lute)'. I first read the last of these many years ago in A. C. Graham's Poems of the Late T'ang and was intrigued by one particular line: 'On Blue Mountain the sun warms, a smoke issues from the jade.' Graham explained this as a reference to Dai Shulun (732-89) who 'said that the scene presented by a poet is like the smoke which issues from fine jade when the sun is warm on Blue Mountain (Lan-t'ien, "Indigo field"); it can be seen from a distance but not from close to.' Although I understood the idea that poetry presents things imprecisely, like smoke on a mountain, and that its richness cannot be studied at close quarters, I was still a bit baffled by the metaphor.
Lantian (Blue Fields) is in Shaanxi province and is most famous now perhaps for Lantian man, the early hominid species. 'In Lantian,' according to Wikipedia, 'white and greenish nephrite jade is found in small quarries and as pebbles and boulders in the rivers flowing from the Kun-Lun mountain range northward into the Takla-Makan desert area.' There is a specific area called Yushan, Jade Mountain, famous for its fine jade. In recent years the Chinese architect Ma Quingyun has established a winery here (as described at some length in an article in The California Sunday Magazine). Back in the eighth century Dai Shulun was saying that in intense heat, the jade hidden in the rocks of Lantian rises into the air. If this is taken literally, I guess he was referring to fine clouds of jade powder from the quarries.
It is possible to go further and read into this line of poetry deeper allusions to its elements: heat, smoke, jade. Just to give one example, there is a story of a girl called Purple Jade who returned after death to redeem the reputation of her lover, accused of tomb robbery. Her mother wanted to embrace her spirit but she just turned to smoke. Therefore, as Maja Lavrač writes in 'Li Shangyin and the Art of Poetic Ambiguity', jade can symbolize something unattainable. Li Shangyin may be alluding in his poem to something or someone attractive but inaccessible. And this is done through a single landscape image that simultaneously alludes to the mysterious beauty of poetry.
There is a lot more to say about Li Shangyin of course - see for example an excellent interview with translator Chloe Garcia Roberts at The Critical Flame. But I will simply end here with her own rendering of this single line of Li Shangyin's:
Indigo fields, sun-warmth,
Jade begets smoke