mom413,

There are a few ways of accomplishing this, depending on the effect you want

On thing I did, with glass, was to use a sander to uncover the glass. I did this because I was seeking a flattened surface. With stones, the tufa should break off in the areas in which it is thinnest. You can use a wire-brush for this purpose, which is what most people use.

Another is to do it in two layers. Here your stones/gems will actually stand out more. What you do is apply one layer of tufa, wait til it dries, remove your mold (upside down plate), apply a much thinner layer of tufa, and add your stones. The layer should be thin enough so as to not cover your stones, and the hardened layer of tufa will prevent the stones from falling in.

Hope this helps.