I used two recipes - neither of which i have cured yet.  Still making the rock. 

Recipe One.

•  1 part - Portland cement type I/II
•  1 part - sand
•  4 parts - oyster shells (rinsed very well)
•  1 part rock salt (salt water softener salt - this will melt in the curing and make it more porous I am told.)

Mix this in to a pasty substance that is similar to cottage cheese or oatmeal.  Use rubber gloves when working with this stuff - it's not good for your skin.  Once mixed i used Rubbermaid containers with sand in them to make a mold.  Be creative here.  My first batch looked like squares and were very obvious man-made.  What I have done to make them less "square" is grab a handful of mix and squeeze it and let it plop into the mold stacking little by little in the mold.  The more unique it looks the better.  After you fill the mold to your liking cover it with more sand and then let it sit for a couple days.  This was difficult for me as I wanted to see what the rock looked like and my second and third batches crumbed because I wasn't patient.  Below are some of the pics of my rocks.  I have more coming.

Recipe Two. This is the whiter of the two rocks

•  4 parts white mortar with sand mix
•  1 part oyster shell (rinsed)
•  1 part rock salt

Mix until it is pasty.  Should be about 1 part water or a little less.

I use 5 gallon buckets to mix mine.  This give me enough mix to make 3-4 smaller rocks and 2 larger rocks. 

For the caved rocks I used balloons and rubber gloves blown up.  Then pop the glove when the cement is hardened and pull out the balloon.