Moss,
Here is an old thread I started
http://www.tractorbynet.com/forums/rural-living/183092-started-making-planters-wife.html
Basically it is a 1 to 3 ratio. The 1 part must be portland cement not mortar. It is called hypertufa and there a loads of info on line. It takes a couple of weeks to cure, must be done slow.
I have made about 400 different pieces, usually do it in the winter since it gets dark early, I can do it in the basement. I actually sold $1000.00+ this year wholesale.
If you really want to give it a try I can give you more info...I have perfected my recipe.
David