Grüner was an educated man, he had studied at the German University in Prague, was interested in Cheb folklore and collecting, and got along very well with the naturalist and poet J. W. Goethe from their first meeting. In the following three years he was his faithful guide and companion during natural history walks, especially the geognostic ones, in the Cheb region. He also visited him in Marienbad, where Goethe spent his last visits in Bohemia. Later, when the poet was no longer able to visit Bohemia, he came to visit him in Weimar and their correspondence continued until the end of Goethe's life. Goethe encouraged him to collect minerals and gave him a collector's specimen box. Grüner started collecting and exchanging with great enthusiasm and after only a year he achieved such results that Goethe nominated him for membership in the Jena Mineralogical Society. He later became a member of the Society of the Patriotic Museum in Bohemia (later the National Museum). After his death his collection passed to the Teplá monastery.
Grüner also acquired a fossil tooth of a mastodon from the owner of several farms in nearby Dolnice, where this tooth had been kept for several generations as a family heirloom. He decided to send it to Count Sternberk as a gift to the collections of the Patriotic Museum and also informed Goethe of his find in a letter. He asked Grüner to postpone sending the precious gift until he had examined it himself and made plaster casts. This molar is the oldest documented find of a Miocene proboscidean in Europe. Goethe advised Grüner to explore the area around Dolnice and to inform the local miners to submit all similar finds for examination. The Dolnice locality and its surroundings were later explored and became a key locality of Tertiary fauna.
