Image courtesy of One Earth Press.
According to Karlsen's research, the type of mineral was never identified in the Sagas, only that it could be used to find the sun and that is was deemed a valuable possession. They were even found listed in the old Icelandic Church asset records.
From St. Olav's Saga: "A stone with which one could see where the sun was in Heaven."
From the Flateryjarbˇk: King Olav (later made Saint) was taught the use of finding the sun with a sunstone.
From the Sturlunga Saga: "When Bishop Godmund and Hrafn Sveinbjarnarson came back to Iceland, after having been one winter in Norway, Hrafn sailed west to Arnarsfjord, to his farm in Eyri, now called Hrafnseyri. But before they parted Bishop Gudmund gave Hrafn a good stud horse and a sunstone."
McGrew translation, as related by L. Karlsen.
Read more in Leif K. Karlsen's book "Secrets of the Viking Navigators."