Structural geologists use Mohr diagram to evaluate stress on rock's surface, but I find it difficult to understand how this diagram works (Though I am good at math). Where can I find a good explanation of Mohr diagram?
Here is an excellent explanation of the math as related the stresses represented by the Moh'r circle.
The best way to start understanding Mohr Circles is this Youtube Video about stress on an inclined plane:
From there, check out the other videos of this guy. But right: this is an open-end question.