The (945) Old Book of Tang biography of Li Xun (tr. Davidson 1950:247) says this occurred on a hot summer day and the emperor's ruyi present was made from "heat-repelling rhinoceros horn", which is believed to be cooling in traditional Chinese medicine.
The Old Book of Tang biography of Li Xun (translated by Davidson in 1950:247) describes this event as occurring on a hot summer day, and the emperor's gift was made from rhinoceros horn believed to have cooling properties in traditional Chinese medicine.