Sunstone (medieval)

The sunstone (Icelandic: sólarsteinn) is a type of mineral attested in several 13th–14th century written sources in Iceland, one of which describes its use to locate the sun in a completely overcast sky. Sunstones are also mentioned in the inventories of several churches and one monastery in 14th–15th century Iceland. A theory exists that the sunstone had polarizing attributes and was used as a navigation instrument by seafarers in the Viking Age.

A stone found in 2013 off Alderney, in the wreck of a 16th-century warship, may lend evidence of the existence of sunstones as navigational devices.

One medieval source in Iceland, "Rauðúlfs þáttr", mentions the sunstone as a mineral by means of which the sun could be located in an overcast and snowy sky by holding it up and noting where it emitted, reflected or transmitted light (hvar geislaði úr honum). Sunstones are also mentioned in Hrafns saga Sveinbjarnarsonar (13th century) and in church and monastic inventories (14th–15th century) without discussing their attributes. The sunstone texts of Hrafns saga Sveinbjarnarsonar were copied to all four versions of the medieval hagiography Guðmundar saga góða.

The description in "Rauðúlfs þáttr" of the use of the sunstone is as follows:

Two of the original medieval texts on the sunstone are allegorical. Hrafns saga Sveinbjarnarsonar contains a burst of purely allegorical material associated with Hrafn’s slaying. This involves a celestial vision with three highly cosmological knights, recalling the horsemen of the Apocalypse. It has been suggested that the horsemen of Hrafns saga contain allegorical allusions to the winter solstice and the four elements as an omen of Hrafn’s death, where the sunstone also appears.

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