Iceland spar, possibly the Icelandic medieval sunstone used to locate the sun in the sky when obstructed from view.
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.