Fire & Concrete mock rock / can I do this?

I seen this sweet fireplace http://davesgarden.com/guides/ls/view/52/ and thought "how about making a raised oven or B.B.Q.?" and wondered if anyone has tried doing this. Starting with the concrete mock rock/using old sidewalk and giving it the "rock look" shown in the picture. Couldn't I make them throughout the winter, storing them in the garage. How well would it hold up?
So before I start making any real plans or doing anything crazy....... I'll welcome Any thoughts and opinions ~~~~ in other words ~~~~HELP!
If you know of any hyperlinks, that would be helpfull too, thanks.