BASIC CLASSIC HYPERTUFA
1 part portland cement
1 part sand
1 part peat moss
Enough water to make a very stiff mix.
Here's two of the classic grot-making site urls: 
Efildoog (scroll down to the instructions)
http://www.efildoog-nz.com/hypertufa.htm
Wylee's famous grotes...
http://www.wyleesee.50megs.com/whats_new.html
Do unto others as you would have them do unto you...
http://wrightspecialties.blogspot.com/
http://vimeo.com/2163435