From the official website www.wieliczka.com.pl :
"Salt was discovered in Wieliczka in mid-13th century in the times of reign of Boleslaus the Modest, the senior prince of the Krakow - Sandomierz Principality, and his wife, Kinga. According to the legend, Kinga, the daughter of a Hungarian king, Bela IV, before having married the Polish king, threw her engagement ring into a shaft of the Maramures salt mine, and the ring travelled miraculously with the salt rock deposit to Wieliczka. Miners drilling a shaft in the place indicated by the princess found salt. The princess's engagement ring was found in the first block of salt delved out. Since then Kinga has been considered patron of salt miners. She was beatified in 1690. In 1999 Pope John Paul II declared her saint during his pilgrimage to Poland."