Ancient tales of Norse mariners using mysterious sunstones to navigate the ocean when clouds obscured the Sun and stars are more than just legend, according to a study published Wednesday.

Over 1,000 years ago, before the invention of the compass, Vikings ventured thousands of kilometres from home toward Iceland and Greenland, and most likely as far as North America, centuries ahead of Christopher Columbus.

Evidence show that these fearless and fearsome seamen navigated by reading the position of the Sun and stars, and through an intimate knowledge of landmarks, currents and waves.

But how they could voyage long distances across seas at northern latitudes often socked in by light-obscuring fog and clouds has remained an enigma.

Enter the sunstone.

While experts have long argued that Vikings knew how to use blocks of light-fracturing crystal to locate the Sun through dense clouds, archeologists have never found hard proof, and doubts remained as to exactly what kind of material it might be.

An international team of researchers led by Guy Ropars of the University of Rennes in Brittany, marshalling experimental and theoretical evidence, says they have the answer.

Vikings, they argue, used transparent calcite crystal -- also known as Iceland spar -- to fix the true bearing of the Sun, to within a single degree of accuracy.

This naturally occurring stone has the capacity to "depolarise" light, filtering and fracturing it along different axes, the researchers explained.

Here's how it works: If you put a dot on top of the crystal and look through it from below, two dots will appear.