What does Li Shangyin's Jinse express the unattainable ideal realm?

Mermaids shed pearl-like tears on the moon-green sea, and the blue fields breathed their jade to the sun.

Several allusions are mixed in the previous sentence of the Necklace. Beads are born in mussels, and mussels are born in the sea. Whenever the moon is bright and quiet, mussels open their mouths to raise pearls, pearls get moonlight, and aurora begins to shine. This is a beautiful folk tradition. Tears, like beads, are natural since ancient times. Jiao Ren's tears turned into pearls, which is also a strange sight in the sea. In this way, the bright moon falls between the seas, and pearls are bathed in tears, forming an indistinguishable fairyland in the poet's pen. It is rare to have such rich connotations and wonderful associations in one stroke.

The blue sky and sea in the latter sentence are not groundless. Si Kongtu, a poet in the late Tang Dynasty, quoted a passage from Dai Shulun who was earlier than him: "The beauty of a poet, such as the warmth of Lantian and the essence of jade, cannot be placed in front of us." The eight words used for metaphor here are exactly the same as the seven words in the next sentence of this poem, which shows that this metaphor has another source. Unfortunately, the ancient books were lost later, and it was difficult to find the source again. It is hard to say whether it is appropriate to quote Dai language as an explanation. Lu Ji, a writer in the Jin Dynasty, has a famous sentence in his "Wen Fu": "Stone is like jade, mountains are only light, and water is like pearls." Lantian Mountain is a famous jade-producing place in the southeast of Lantian, Shaanxi Province. This mountain is sunny and contains jade gas (the ancients thought that treasures have phosgene invisible to ordinary eyes), and Ran Ran rises. However, although the essence of Meiyu is far away, it is not close, so it is out of reach, representing an extremely beautiful ideal scenery, but it is impossible to grasp and approach.