"A piece of ice in a jade pot" means: ice in a jade pot, which is a metaphor for people's integrity. It means my heart is as white and transparent as ice in a jade pot.
Source: Farewell to Xin Jian at Furong Tower
Tang Wang Changling
Enter Wu in the cold rainy night,
Send off guests in Chushan in the bright morning solitary.
Relatives and friends in Luoyang ask each other,
A heart of ice is in a jade pot.
I came to Wudi at night when the cold rain filled the river sky. After seeing off my friends at dawn, only the lonely shadow of Chushan Mountain was left.
When you arrive in Luoyang, if any relatives or friends ask you about me, please tell them that my heart is still as pure as the ice in a jade pot, untainted by fame, wealth and other worldly sentiments. Extended information
This group of poems was probably written in the first year of Tianbao (742) when Wang Changling came out as the county magistrate of Jiangning (now Nanjing). Wang Changling became a Jinshi in the fifteenth year of Kaiyuan (727); he was banished to Lingnan in the twenty-seventh year of Kaiyuan (739); he returned to the north the following year and served as Jiangning Cheng from the end of the year, still a relegated eunuch. Xin Jian was a friend of Wang Changling. This time he planned to cross the river from Runzhou (now Zhenjiang), via Yangzhou, and go north to Luoyang. Wang Changling may have accompanied him from Jiangning to Runzhou and then parted ways here. These two poems were written at this time.
The word "lone" is like a lead of emotion, which naturally leads to the last two parting words: "Friends and relatives in Luoyang are like asking each other, and a heart of ice is in a jade pot." The poet started from the clear and flawless, clear sky. He held out a crystal clear and pure ice heart from the bottomed jade pot to comfort his friends. This could express his deep love for his relatives and friends in Luoyang better than any words of lovesickness.