Iolite: gem of the Vikings
When Leif Eriksson and the other legendary Viking explorers ventured far out into the Atlantic Ocean, away from any coastline that could help them determine their position, they had a secret gem weapon: iolite. The Viking mariners used thin pieces of it as the world's first polarising filter. Looking through an iolite lens, they were able to determine the exact position of the sun, and navigate their way safely to the New World and back.
http://slurl.com/secondlife/joia%20azul/114/128/25
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