Hi Jimmy,
This is my suggestion. I've heard about it, but never tried it. Perhaps other readers can offer some insight.

First, buy a roll of bubble wrap- the large bubbles, not the small. (You can find it at an office supply or shipping and packaging supplier.) Then find a spot in your yard that is clay and dig a hole the shape and size you want your rock. Line the hole with a layer of the bubble wrap. Mix up some cement and pour it into the hole, pushing it against the bubble wrap. If you want the rock to be a little lighter, your can place a block of styrofoam or other lightweight material in the center. (You can also add vermiculite to the cement.) Let the cement harden for a few days, then dig around it and flip it over. Peel off the bubble wrap and "paint" the rock with sour cream, cottage cheese, or other dairy product. Place it in the shade and the rock with be covered with moss in a month or two. It'll look really cool because of all the impressions the bubble wrap left.

Give it a try and send me some photos! DonRawson@juno.com

Go to the hypertufa forum. With a little reading, you'll find all the info you need.

check out this, you can order the DVD, very good instructions

www.keithtutor.com