Back in the day, the outside of the Pyramid was covered in polished, shiny limestone which made it sparkle. This limestone was given cuts to make it appear flat under direct sunlight.
How did it come to be the way it is today? It is said that there was a massive earthquake in 1303 AD which caused some stones on the outside to loosen and crack; these were eventually reused to make fortresses and mosques leaving the sand core of the Pyramid exposed.