Part II -- Stone, Marble, and Cracks
The biggest problem I've really found with cracks is that people put the highlights on the wrong side. Decided to explain it with physics.
Hope it makes sense...
As always, feel free to ask questions!
Part I : Clouds, Trees and Technique
Part II: Marble, Stone, and Cracks
Part III: Skin and Face
Part IV: Details, and Clothes
Part V: HairHow to Learn from Tracing Final Image