I took a look at your hand painted materials, and you are on a good track. The highlights are nice touch. I have a few notes for you here, and I encourage you to keep going. :)
Hand painted Brick
Cracks and Detail
The very first thing I noticed was the nature of the cracks. They are very uniform and lack a kind of organic feel. You'll notice after I highlight them, you'll see you have only 2 major angles. Lets brake them up.
Here are a couple of crack references I pulled really quick from a quick google search. it's really so much better if you can pull form life, but as long as you are looking at the world.
notice how the cracks kinda star out and fractal? you started to do that in a couple of bricks, but could have used the star pattern to express an impact.
Pulling from life is key. I'm going to apply what I demoed above to your material:
So, this part is a matter of taste. You are developing a style of your own, and there are SO MANY ways stones crumble. so this is really just some extra ammo in you arsenal:
You've Chosen a monochromatic color scheme. This is perfectly fine. Lets take a look at the color :)
The "Goldie Locks zone", isn't a definitive concrete rule. The shades and saturations outside of that zone don't typically happen on objects in nature. . So when you are learning to render from life, it's a good constraint to give yourself..
However, you lighting falls a little flat. just a little. usually there is color in light. So here's a little brake down/paint over I did for you
I blocked in some varying heights of the block and added a little terracing on a couple. I think I just like how those look,. That isn't ALWAYS applicable. It really depends on the reference you are pulling from. but i like it, so i did it.
I reinforced the blocking and added some shade to the blocks that are "lower" and pushed the values of the cracks and where there would be darkness.
Like I said, you are on a good path. You are working at it, and it shows. Beyond what I've gone over, i would encourage you to do some texture studies as well. This is some of the hardest part, but if you can lock it down, you'll be pretty undstoppable. :P
Artist - http://mittmac.deviantart.com/
I can maybe go over some more sometime, but i got a meeting and a ton of work today. :)