In this task, you're given a passage, further information available on a particular linked term from the statement, and a question. Your job is to generate the answer to the question by using the information provided. If there is no clear answer obtainable, output 'none'.
Q: Passage: Birks, a company that designs, manufactures and retails jewellery, timepieces, silverware and gifts, acquired the building in September 1912. The building was significantly reworked in 1912 to accommodate the jewellery store. The rework added distinctive Renaissance Revival palace facades designed by Percy Nobbs, featuring terracotta, granite, bronze and Tyndall stone. Above the third-floor openings are six terracotta medallions depicting the sources of the materials used by jewellers, with a seventh medallion on the north facade. These medallions depict turquoise (representing semi-precious stones), an elephant (representing ivory), a Kimberley Negro] searching for diamonds, a man diving for pearls, an oceanic wave delivering the riches of the sea (mother-of-pearl, coral and a tortoise shell), a precious metal-smelting gnome, and a silversmith surrounded by the tools of his trade. Above the medallions is a frieze depicting such characters and places as King Solomon, the Queen of Sheba, gates of Jerusalem, Hiram, king of Tyre, Negroes and an Indian, and the three wise men giving and receiving gifts. $150,000 of alterations to the ground-floor show-window area in 1951 included a granite base and Tyndall stone facings surrounding the solid bronze show windows, as well as corner columns and vestibule walls lined with Travertine marble. The building was the Winnipeg showpiece for Birks for nearly eighty years. By 1991, the basement, first, second and third floors had all been substantially altered by the Birks Company, leaving only the fourth floor of dormitories unaltered from the YMCA era. Birks continued in this building until the 1987 when it moved to Portage Place.
Link Information: Terracotta is the term normally used for sculpture made in earthenware Question: Which of the materials that the palace facades featured was naturally formed?
A:
Answer: Tyndall Stone