For mold type rocks the the fiberglass fibers or OK I guess although you can see them in the surface. These are normally only used in subsurface layers where a finish coat is appied on top. For free form rocks I don't use them because they stick out and get in the way. You can buy fiberglass fibers at any mason supply.
I don't use any reinforcement for rocks formed in place. They have to have some thickness, like 1" or 2" at least. More mortar is used but it's easier and it's not like I'm shipping them.
I'll feed your growing addiction...Dig a hole in the ground as a mold and just slather the mortar against the sides of the hole. Very organic looking rocks. You can also damped the soil, smooth the dirt a bit and then add mortar to get smoother rocks.
Do the dirt hole mold in a rectangle shape. This mold will be filled solid with mortar. To fill put a bit of mortar into a bucket and add coloring. Put that into the mold. Add another bit of mortar to the bucket and a different color. Repeat until mold is filled. Let the mortar setup, maybe an hour until firm. Take it out of the hole and crack it into slices. Gives you a veneer stone with wild colored layers.
For big rocks the best method is free form. Wire mesh, crunched up to form the shape. Apply a thin layer of mortar. It can be difficult to do the whole rock at once on your first try. When you giggle the wire the previous mortar tends to falls off. As you get the viscosity right with experience, and a lighter touch, you can do a whole rock at once. When done let the mortar setup for a day or two. Now you'll have a very stiff structure to apply the finish layer. These huge rocks can be filled with soil and plants added through little holes which you form when making the rock. Form a little dish too so water can collect to make watering easier. These are my fav rocks to make. Use stainless steel mesh if the rocks are used inside the pond or water feature. I also use plastic mesh fencing because I'm cheap, but it only works for plat type rocks. Other wise regular mortar mesh can be used. Cheaper chicken wire can be used too, but harder to work with.
As others have already said, no freezing conditions. If you want to work in wet cement in 20 degree weather you should try it. If you still want to after that you should see a doctor as you have serious problems.
