Cob: Rock Staining Recipe?

[Douglas Ketler Scheffer]

Inspired by a piece of information from Robert Boleman's slide show 
several months back, I have started building a dry stacked wall out 
of "Urbanite" (recycled chunks  of of concrete) near my patio garden.

This stuff creates a rather beautiful appearance for the price, 
especially when the chunks are uniformly stacked so the inside edges 
make up the exposed sides of the wall. The little stones imbedded in 
the concrete make each chunk look rather natural. This is a great way 
to make use of an abundant resource that is normally discarded, and 
it's usually delivered to your door for free.

I have seen similar walls like this where the builder stained the 
wall with some kind of iron or ferrous solution and then had lots of 
green grass growing between the stones. I think this staining 
technique adds a beautiful warm & rustic look to the material.

Does someone on this list know how this staining is accomplished? 
Needless to say I want to stay away from anything that is potentially 
toxic.

Thanks,
dpks