sunstone. 1 Crystal mineral thought by some to have been used by the Vikings as an aid to navigation, especially in conditions of low visibility due to clouds or fog when the position of the sun was uncertain. Both Iceland spar and cordierite, which have optical properties that cause rays of light to show different properties in different directions, thus permitting an observer to determine the position of the sun, have been suggested as candidates for the sunstone. 2 A reddish or golden form of a plagioclase feldspar that is used as a gemstone.

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