Dear Doug, first of all, congratulations on your Italian.
My message (n.3354) was divided into two parts.
In the first part of the message I specified where I had heard the sunstone mentioned for the first time and where I had seen it (so movies and TV series).
The last of these TV series (the one I remember well) is the fictional series about the Vikings (I will put the link at the end).
It is clear that I did not give and do not give any scientific or historical credit to the fiction broadcast by Netflix (yours is a strange question, I was certainly not clear enough in the translation).
No question about the contents of Netflix, we all agree.
However, this is only the latest fiction among the many about the Vikings. The one I remember best because it is more recent.
In the second part of the message I wanted to clarify that I later found that this sunstone was not only not pure invention but perhaps it worked and perhaps it had actually been used by the Vikings.
In this regard, I cited a text from Focus (rather than others readily available to everyone such as Wikipedia) because it seemed concise enough to report it verbatim and because (perhaps) less easily available.
On this subject, I want to open two digressions.
In Italy, the Vikings are known by different terms: vichinghi, norreni, variaghi or vareghi and normanni.
The term vichingo, although the most used and best known, is considered not very correct and is valid only in its meaning of marauders. It should be used only for that part of the population that raided and pillaged.
The term "norreno" is the most correct and includes all those Scandinavian peoples who in a certain period began raids and then invasions in Great Britain, France, Russia and Italy.
Be careful, however, to define the Vikings, the place of origin is not enough, they are not the only ones to come from Scandinavian countries. Already many centuries before, both the Goths (Visigoths and Ostrogoths) and the Lombards had gradually descended towards the south coming from Scandinavian areas (Gotland).
The term "variaghi" o "vareghi" (Varangians) is used to specifically indicate the Norsemen who advanced into Russia, sailing on the great rivers, and who founded the various Rus kingdoms (predominant over the much more numerous Slavic population).
The term "Normanni" (Normans) refers to the Norsemen who conquered southern Italy and Sicily, subjugating the Romans, Lombards and Arabs.
The emperor Frederick II (the stupor mundi) grandson of Frederick I Barbarossa was on his mother's side of the Norman family of Altavilla and Hoenstaufen on his father's side.
Sorry if there should be mistakes but I wrote based only on memories.
Another digression:
Many historical or historically set films have little connection with historical reality, and yet they are beautiful.
I think of Gladiator (the first) and Bravehearth.
A mass of gigantic errors, yet they turned out to be beautiful films.
On the other hand, very accurate films turned out to be ugly, boring, terrible from a cinematic point of view.
A film is not and cannot become a documentary.
https://it.wikipedia.org/wiki/Vikings
https://www.netflix.com/it/title/70301870