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Thanks for your answer berak,

You are right, these cracks/failures can come in any size or shape, but they tend to have a determinated orientation or angle depending of the reason of the failure (shear, flexure, etc), and obviously the crack comes always in gray/grey.

Here is an ilustration of how the shear failures comes, and a picture of a real shear failure.

http://ijripublishers.com/IJRI-Publishers/pdf/CIVIL/VOLUME%201/IJRI-CCE-01-003/3.png

http://alert.air-worldwide.com/Alert/alertdata/Protect/Data/Chile__Earthquake/L3/Fig9_Shear.jpg

As you can see the shear failure tend to come in a 45° angle