sunstone
... that Vikings may have navigated using a light-polarizing mineral they called a sunstone to locate the sun in cloudy skies?
... that <a class="mw-redirect" href="./Viking" id="mwGRs" rel="mw:WikiLink" title="Viking">Vikings</a> may have <a href="./Longship#navigation" id="mwGRw" rel="mw:WikiLink" title="Longship">navigated</a> using a light-<a href="./Polarizer" id="mwGR0" rel="mw:WikiLink" title="Polarizer">polarizing</a> mineral they called a <b id="mwGR4"><a href="./Sunstone_(medieval)" id="mwGR8" rel="mw:WikiLink" title="Sunstone (medieval)">sunstone</a></b> to locate the sun in cloudy skies?