 Later, in section 36 of his "Naturalis Historia" Pliny mentions the exploitation of "lapis specularis", a variety of translucent specular gypsum that was very popular at the time for the manufacture of window glass and that would be for a long time an important part of Segóbriga's economy.This mineral was extracted from mines found in "100,000 steps around Segóbriga," and Pliny assures us that "the most translucent of this stone is obtained in the Hispania Citerior, near the city of Segóbriga and extracted of deep wells.