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.
