James Watt opinion 11/6/1987: Song
Xia Nai opinion 1986: Tang or Song
Thomas Lawton opinion 11/7/90: could be earlier, Eastern Zhou. Many Eastern Zhou pieces highly lustrous, but many also (like this one) have non-lustrous surfaces.
A gui is a tablet with a pointed tip. An ancient Chinese text entitled the' Zhouli' (Rituals of Zhou) describes the gui as a ceremonial object to sacrifice to the direction east. It was also used in Daoist rituals.