both.
I'm changing my answer, it is very possible to make your own bricks.
Each brick is made using a custom file that is scripted. For basic bricks, under this file you have generic dimensions, collision units, and then quadratics scripted out for each face of the brick. as well as texturing information for the brick.
I'm still working on messing around with the .blb files to see what all controls what on some of the special shapes, but by following badspots tutorial stuck to the top of this section, creating your own bricks shouldnt be difficult.