I think you're confusing with the calcite found on the 16th century British warship. The article mentions: "None have ever been found at Viking archaeological

I think you're confusing with the calcite found on the 16th century British warship. The article mentions: "None have ever been found at Viking archaeological sites, but a calcite crystal was discovered in the wreck of a British warship from the 1500s, indicating it might have been a tool known to advanced ocean navigators." ... but this is wild conjecture at best. None of the written sources of that time (or, in fact, later) mention the use of calcite for navigation.