The cornerstone?

http://www.youtube.com/watch?v=ETO3-MxMe2Q

The cornerstone (or foundation stone) concept is derived from the first stone set in the construction of a masonry foundation, important since all other stones will be set in reference to this stone, thus determining the position of the entire structure.

Over time a cornerstone became a ceremonial masonry stone, or replica, set in a prominent location on the outside of a building, with an inscription on the stone indicating the construction dates of the building and the names of architect, builder and other significant individuals.

In ancient buildings, the foundation stone was placed at the north-east corner of the structure.[citation needed] This was thought to be an auspicious position.

But, what does it mean, when a cornerstone appears in a Spiritual book?

A cornerstone is the most important/significant stone.

That is to say, that the foundation for humanity was something not of the builders choice. And likely this forshadows the downfall and crumbling ephemeral nature of any accomplishment of man, just as a house built upon sand crumbles to the aether flow of the desert sea.

The house of man must be completely rebuilt in order to correct its imperfections. A man must let go of everything he has built himself upon to be in the builders favour.

.... I could spout out these metaphors all day, haha

Life's a journey not a destination
And I just can't tell just what tomorrow brings.
It's a long, long road
From which there is no return.