It goes without saying that Qianlong period (1736-1795) collectibles are among the most coveted in the markets, and regularly sell for huge amounts at the world's top auction houses. The latest piece set to continue this trend is an Imperial spinach green jade double-gourd 'San Xi Tang' seal.

The rare and important seal is estimated to sell for more than £1m ($1,577,640) at a Fine Chinese Art sale in London on May 17. It has been authenticated by Guo Fuxiang of the Palace Museum, Beijing, the leading Chinese academic in this field.

According to auctioneer, the rare double-gourd shaped seal measures 8.7cm long. It was almost certainly commissioned, and personally handled, by the Qianlong Emperor himself.

San Xi Tang (or the Hall of the Three Rarities) measured only four square metres and was situated in the Forbidden City, the Chinese imperial palace from the Ming Dynasty to the end of the Qing Dynasty, which is today the location of Beijing's Palace Museum.

The Imperial spinach green jade double-gourd 'San Xi Tang' seal

The Forbidden City was the home of emperors for nearly 500 years. San Xi Tang was an important personal space of the Qianlong Emperor, in which he kept three prized rare scrolls.

They were: Wang Xizhi's Kuaixue Shiqing Tie (timely clearing after snowfall), Wang Xianzhi's Zhongqiu (Mid-Autumn festival), and Wang Xun's Bai Yuan Tie (letter to Boyuan). The three rare scrolls were stored among many other important antiquities.

This seal is carved in an auspicious double-gourd form. As well as representing Heaven and Earth, this double-gourd form was also useful for longevity. The seal's upper section is carved with three chi dragons (chilong).

"We are delighted to have brought to light this important and long lost Imperial seal," said Asaph Hyman, Director of Chinese Art at Bonhams.

"Until this moment only the impression of the seal was recorded in the Imperial archives, but now academics and distinguished collectors can study and cherish the actual work of art."