Human Enamel, World's Tiniest Sandwich (IMAGE) Northwestern University Caption The "world's tiniest sandwich." An atomic resolution scanning transmission electron microscopy image of an enamel crystallite looking down the long axis of the crystal. The dark areas show magnesium ions forming two layers on either side of the core. Credit Northwestern University Usage Restrictions Credit must be given License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.