The jewel of the Jász Museum is the Jász horn that was began to be called Lehel's horn after the saga spread in the 17th century. It was the horn serving as the badge of dignity. After the chief was captured it was most probably stolen. Throughout centuries, many horns were believed to be Lehel's horn. Until the 16th century, it was called the horn of the Jász. It is still not proven that the horn in Jászberény is the real Lehel's horn.

According to the present state of scientific research, the 43-cm long horn made of ivory was made in Kiev in the 12th century. It is possible that artisans who knew about the stylistic characteristics of the Byzantine made it, but they modified the style according to local needs. The ornamented horn was probably given to one of the Jász leaders through the Russian-Jász relations and was brought to Hungary by the people who moved to the country.

The Jász horn is the most important relic of the Jász people. It was the dignity badge of their captains. At time of celebration, they had their favored quests drink of the horn. For the Jász, the horn was the most important symbol of the sense of belonging. It was presented on stamps, coats of arms, flags, but it was on many charts and objects as well.

The horn was mentioned as instrument in historic works. For a long time a tale was spreading saying that it was Chief Lehel who last blew it and there is no man who would be able to sound it again.

The horn hung on the wall of the church of Jászberény in the 17-18th century. From the beginning of the 19th century it was kept at the Town's Hall, from 1874 it was entrusted to the Jász Museum. The belief that the horn has a mythical power is well known since 'whoever had a drink of the water of the river Zagyva from the horn became a Jász himself'.