The Archives preserve, for instance, the most ancient monument of the Mongolian language in our possession is the so-called «stone of Genghiz – Khān» kept at St. Petersburg, engraved around 1225 to commemorate a bow contest on the occasion of a diet held in Eastern Mongolia; the second one is the seal of Güyük – Khān put twice on the famous letter of 1246 that Giovanni da Pian del Carpine took to Innocent IV the following year (only the seal bears Mongolian characters, the letter was written in Persian, with a preface in Turkish and the date in Arabic).

http://asv.vatican.va/en/arch/mongolian.htm