Image courtesy of One Earth Press .
Because of the nature of the calcite crystal (see section on calcite optics), these rhombs all show the extreme birefringence employed by Karlsen in his working model. This would have been evident to anyone picking up one of the bright pieces; an experienced navigator could readily divulge the significance by observing how the images changed in the light.
Karlsen and his wife June travelled to Iceland to see for themselves how easy it was to find these crystals in the scree and were successful in observing many which were suitable for use in navigation. Iceland Spar is now protected in Iceland and it is forbidden to remove anything from the old mine sites. Read more in Leif K. Karlsen's book "Secrets of the Viking Navigators."
For a historical perspective on Icelandic Spar calcite and its impact on science, please see the delightful paper written by Dr. Leo Kristjansson on "Iceland Spar: The Helgustadir Calcite Locality and its Influence on the Development of Science" (pdf file, 559 KB reproduced with permission of the author). A more recent and vastly updated paper in Icelandic will be available soon in English.