The final polishing of leaf gilding, today done with an agate, formerly done with a hard stone like haematite or an animal’s tooth. To be burnished, the metal leaf has to be laid by water gilding, i.e. on bole mixed with size or glair, not on an oil mordant.

Source: Skaug, Erling. Punch Marks from Giotto to Fra Agelico: Attribution, Chronology, and Workshop Relationships in Tuscan Panel Painting: with Particular Consideration to Florence, c.1330-1430 (Oslo: IIC, Nordic Group, the Norwegian section, 1994), p. 12.