Objective: The objective of this study was to compare the microstructures of teeth, natural opals, and opal dental enamel porcelains to examine the mechanisms that cause opalescence. Method and materials: Four dental porcelains, a natural opal mineral, and a human tooth were examined. Replicas were assessed in transmission electron microscopy for features that would cause opalescence. Enamel dental porcelains denoted as "light" were selected for the study. X-ray diffraction and color analyses of the porcelains were also performed. Results: All the materials were opalescent, to varying degrees. The porcelains with fewer microscopic features were less opalescent. The presence of dispersed particles or a phase-separated glass was found to cause opalescence in dental porcelains. Conclusion: A phase-separated glass in one enamel porcelain best resembled the microstructure of natural opal mineral and teeth, and this was the most "opalescent" material.