I'll also likely be buying another set of silicone molds from a different company. They're more expensive, but there are also a lot more of them in the batch, so it will give me a lot more variety in shape and texture. And I'll likely make a few of my own in the process to cover some odds-and-ends pieces. More than anything it's to have a large batch of individual stones that I can make at one time. Mixing and pouring the concrete is the easy part.
The mix is interesting, too. There are a lot of materials that you can add-- funny enough, they're called admixtures-- to give your concrete different properties. For stone veneer, light weight and durability are pretty high on the list. And it's a different mix for exterior stone than interior stone-- exterior needs more durability to stand up to weather.
Coloring the concrete is the arty part. There are two real parts to the process: a base color that's added to the concrete mix wet, and random pigmentation which is sprinled or brushed into the mold dry before the concrete is poured into the mold. It pigments the surface to give a more real, natural look.
You also have to seal the stone veneer. There are sealers made specifically for that purpose, with different finishes from invisible to matte to high gloss.
There are also molds for tile available. Everything from granite to slate to cobblestone. And the look is amazing-- it's not the pressed concrete look, it actually looks like real stone tile, probably because it's individually installed tiles and there are random variations in each. All the stamped concrete that I've seen looks like... well, stamped concrete. Cracks like it, too.