Kangxi seal of the "Mandate of Heaven". [Photo/China Daily]

A sandalwood seal carved for China's longest-reigning emperor will be auctioned in Hong Kong on April 6.

The "Mandate of Heaven" seal belonged to Qing Dynasty (1644-1911) Emperor Kangxi who ruled for 61 years. It bears an inscription saying "revere the heaven and serve thy people".

The seal will appear in Sotheby's salesroom. Also to go under the hammer in the same sale is another seal of the emperor carved out of the famed Shoushan stone from Fujian province, and Kangxi Baosou, a complete record of his seal impressions.

The record was commissioned by the emperor's grandson, Emperor Qianlong, as an expression of piety. Two copies were produced. Besides the one that will be sold from a private collection, the other one is at Beijing's Palace Museum.