This is a DIY rock tutorial I made...
http://www.youtube.com/v/hboNJHlgOvc&hl=en_US&fs=1&
http://www.youtube.com/v/wndkovV8DpU&hl=en_US&fs=1&
http://www.youtube.com/v/cgI3aiMCJyk&hl=en_US&fs=1&
http://www.youtube.com/v/We95IFct7R8&hl=en_US&fs=1&
http://www.youtube.com/v/OXxyvNZJOaY&hl=en_US&fs=1&
Some things of note.
-With the formula I have you need to let the molds sit for 48 hours to cure properly. If you want it stronger (also it will be less porous and heavier) you can remove the salt from the formula or add extra cement.
-You can't make things too thin or they will brake.
-Will flake lots of stuff off (using more cement stops that)
-You can use sand as the medium for molding your rocks in.
-If you use balloons for making shapes you need to make sure you take out the balloons when done. (some times the balloons pop and you have to chisel the balloon out or just through out the rock)
-The oyster shell can be substituted by aragonite sand or crushed/pieces of coral. (It is not anywhere near as porous but is way stronger... but I would prefer to use no salt or more cement then aragonite or crushed coral)
-After being made it needs to soak for up to 2 months to make sure most of what can leach out has leached out. (with a proper set-up you could possibly do it in 1 month or less with the formula I have used as there is lots of water flow through the entire structure... a downside of that is there is much less anaerobic areas in the rock turning nitrates to nitrogen... most tanks don't try to do that process anyways, on the upside there is lots of aerobic area for turning ammonia/ammonium all the way into nitrates... and that's the most important part of the nitrogen cycle in most tanks.
Formula:
-3 Parts washed crushed oyster shell (can be picked up at almost all feed stores, very inexpensive... in a city you might have to go to the edge of town as not many people in cities keep chickens
)
-1 Part Aragonite sand (picked up at most fish stores can be substituted for pool sand or removed from the formula all together... I like to keep it in there)
***Edit*** (now I would not use less then 1.5 parts of White Portland Cement, it needs it for the strength and to prevent it from degrading as fast in the water.)
-1 part White Portland Cement type 10 or general use (can be picked up at some places where cement supplies can be purchased. You can use the gray stuff as it is cheaper and easier to find, but I have heard that it takes longer to cure and it doesn't look as good IMO, also to make your rock stronger you can add extra cement to your mix... makes it slightly less porous and slightly heavier... would also make more anaerobic areas)
- 1 part Course Rock Salt. (I used water softening salt it can be purchased at some grocery stores as well as at some appliance stores and some hardware stores... also very cheap... can be removed from formula to make rock stronger... also makes it less porous and heavier... also makes slightly more anaerobic areas)
I put my DIY rocks in the ocean on the 20th and will be pulling them out July 11th.