They are often referred to as 1000 year old eggs or 100 year old eggs, but are not nearly that old. They are in actual fact, simply preserved duck eggs. The inner yolk part is firm and rich in texture and the outside is similar to jelly.
Sometimes, the inner part is somewhat runny and I always thought it was because some preserved duck egg yolks were meant to be that way. However, I discovered that if the egg is boiled for 8 minutes or so, then the egg would firm up, both on the outside and the inside, similar to a hardboiled egg.