opal glass that is stronger than regular stoneware. Internal tests show Ultra-Dura out performs other stoneware dishes by as much as 3 times in terms of impact resistance and 4 times in chip resistance. The high fired glaze creates a more hygienic, fully vitrifies surface with almost zero porosity. This set is dishwasher, microwave, and freezer safe for easy storage, clean up, and reheat. Comes with service for four.