Pyramid of King Amenemhat III
The Pyramid of Amenemhat III (often referred to as the 'Black Pyramid') is located in Dahshur, in Giza. Now, the pyramid resembles a mound of rubble. Its core was built of mud bricks and encased in limestone. Most likely, the reason why it collapsed is because it lacks a stone core.
The pyramidal granite cape stone is now exposed in the Egyptian Museum.