Century eggs (or pei dan in Cantonese) are eggs that have been preserved in a clay and salt mixture for several weeks to several months. The result is a dark green or gray yolk and black translucent jelly-like exterior. The flavor of the yolk is strong and sharp, similar to ripe cheese with some sulfur flavor, and the texture is creamy, making it a great addition to congee. Duck, quail and chicken eggs are all used in this type of preservation. You can purchase century eggs online and at most Chinese markets. Some markets sell century eggs as "preserved eggs"; make sure the egg is black translucent.