I used the plastic store bought molds. I cut a piece of contact paper to fit the bottom of the mold. Use the clear contact paper. Draw your design with permanent marker. Remove the paper backing to reveal the sticky side. Put it in the bottom of the mold with the sticky side up,
Now use your glass, stones, etc. to fill in your design outline. Mix your concrete and slowly spoon over your design and gradually fill to the top,
Let it cure 24 hrs or so and then remove from the mold. Turn over so the contact paper is on top, Slowly pull off the contact paper from the concrete and like Dave, clean up any concrete film that may have seeped onto the design. Finish curing in water as per Dave's instruction.
I have lots of scrape stained glass and plan to make more mosaic stones when I have some spare time.

That's another great technique, Jan! I like it!

The advantage to using your mold Dave is that you can cut the contact paper a bit larger and use the weight of the mold to hold down the edges. I have found that sometimes the contact paper wants to curl up on the edges. If that happens, i spray some PAM cooking spray in the mold before putting in the contact paper. It acts as a temporary glue to hold the contact paper in place until the concrete is put in. Off course it also helps to release the stone after curing.

What a timely article! Like Jan, I have quite a bit of scrap stained glass and have been wanting to make some stepping stones with it for my garden areas... I'm planning to use the contact paper method, which I read about somewhere or other in the course of researching this project, and Dave's mold-making technique. Dave, your stones look beautiful, and I suspect the photos can't really do them justice -- your article has inspired me to stop thinking and start producing!

I'm glad you're inspired!

I'm going to try this contact paper next time we make stones. I bet it'll work great.

When using glass, remember that the pieces of glass have to be mechanically held in place, like Dave said. If you are using rocks or shells or broken pottery it is not as critical.

The reason for this is in how concrete works. Concrete hardens by growing microscopic needle shaped crystals that stick into the rocks and stones and holds them in place. Rocks and stones have tiny holes or pores that the crystals grow into. Glass doesn't (or has very few). That's why the glass eventually comes loose. The tumbled glass has had the surface roughed up so the concrete has a little more grab

If you want to use flat pieces of stained glass, bevel the edges of the glass so that concrete can grab it. Leave some concrete in between the pieces of glass.

Why soak the stepping stones before using them? Concrete continues to slowly grow the crystals as long as the concrete stays moist.

Why a month? Concrete gains the main portion of it's strength in 28 days.