Glass substitute: lapis specularis

The recent discoveries involving the Roman Amphitheater in Mérida, Spain have been making the news. One of the interesting discoveries was a Roman window grill (pictured above). It is thought that this grill was used to hold translucent stone (known as lapis specularis) instead of glass.  Lapis specularis was used because it is a cheaper, more readily available substitute that could be mined from local sources.
 

(A modern example of lapis specularis)

(A window made from lapis specularis in the Roman forum of Cartegena)

Lapis specularis was the Roman name for a translucent selenite gypsum stone that can be cut out of the local mines in large crystal-like sheets and used as window panes.

According to Pliny the Elder, lapis specularis could be found throughout the Empire, but the best source of clear and large pieces of lapis specularis were found near the modern city of Segorbe (ancient Segobriga), Spain.

Quote

 

(Pliny Natural History, Book 36, chapter 45);

As to specular stone—for this, too, is ranked as one of the stones—it admits of being divided with still greater facility, and can be split into leaves as thin as may be desired. The province of Nearer Spain used formerly to be the only one that furnished it—not, indeed, the whole of that country, but a district extending for a hundred miles around the city of Segobrica But at the present day, Cyprus, Cappadocia, and Sicily, supply us with it; and, still more recently, it has been discovered in Africa: they are all, however, looked upon as inferior to the stone which comes from Spain. The sheets from Cappadocia are the largest in size; but then they are clouded. This stone is to be found also in the territory of Bononia, in Italy; but in small pieces only, covered with spots and encrusted in a bed of silex, there being a considerable affinity, it would appear, in their nature.

 

(Evidence of Roman mining for lapis specularis in Segóbriga. See video below.)

https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D36%3Achapter%3D45