The information in the tutorial section is a very big help. I found the car tutorial
http://ac3d.org/ac3d/tutorials/carbody/index.html really helpful in learning how to use the texturing tools.
As for creating a diamond, I guess it depends on what kind of diamond you want. You could create a cube, rotate it onto a corner, and stretch the corners out a bit, or if you want something more gemlike, There is the polyhedrons located in the tools menu. I'm sure one of them could easilly be modified to look like a diamond.