The École Polytechnique owns some large Iceland crystals that were once used by its Professors to teach birefringence and optical properties of crystals. Olivier Hardouin Duparc re-uses them in a tutorial video produced in the Main Library of the École to clearly illustrate the phenomenon discovered by Erasmus Bartholin in Copenhagen and immediately worked further by Christian Huygens in Paris with his famous two calcite experiment. OHD also explains and shows the fascinating atomic structure of calcite (CaCO₃ based), showing the connection between its crystallographic unit cell and its well-known morphological shape.