What's This? How Should I Eat It?

When I went to Hong Kong last December, I found a strange food at a street market. It seemed an egg and actually I found it an egg later at the hotel I stayed. 

I didn’t know it was a chicken egg, a duck egg or else. 

It was in a plastic bag written something in Chinese. When buying, I didn’t ask what it was and how to eat it. 

The egg was covered with black clay and ash like powder. It didn’t smell at all. 

At the hotel, I took it out of the plastic bag and washed off the clay and powder carefully. Then a white egg appeared.

I broke the eggshell. … It was a just a normal raw egg.

It still remains in my mind as a question of Hong Kong.

Does anyone know how I should eat it?