I would buy the premade cement stepping stone and go to the dollar store and buy some stones, nuggets, dishes?? If you get them to smash the plates then put in a plastic jug with water just covering the smashed bits, screw lid back on and shake to dull the edges? I'd only use porceline plates tho. You can do the same with stained glass, pre-cut it yourself and dull the edges with the jar or a rock tumbler. Sounds like fun!!!

Try to find some one to donate some tiles or plates .You might be surprice to find some donation .

For my children classes, I start them out with picture frames and things like buttons, glass nuggets, sea shells, sea glass, beads. I would also suggest as Calamity says, you can nip your own and tumble them if you feel that would be safer. Porceline plates or even bath tile are going to be VERY sharp.

Picture frames would be a great idea! Then I could take a picture of each kid working, have it printed, and then put it in the frame when the class is done. And I should be able to find things like buttons and other sparklies at an antique store. I'm just a little too worried about doing things with sharp edges.

Premade stones, and marbles the 1/2 ones! and tumbled glass, with mortor or thin set. I 'd just tumble bunches of glass, and let them go for it! No cutting! just fun!

There are some really pretty colors of ceramic mini tile. You don't have to cut them and they don't have any sharp edges. I have bought them from several places including Ebay.

you might also check out Maryland Mosaics and Mosaic Basics. they both have them. I think Basics has them as low as $9 something a lb which is 450 tiles. But if you put Gardenweb in the code section, you can get 10% off at maryland mosaics!! Just check them all out.

and the ideas of shells, buttons etc all are great ideas. I know you will have

fun http://www.marylandmosaics.com

another idea is the marbles from places like walmart. I bought a big batch of different colors a couple of weeks ago, in the crafts area. They were less than $2 a bag.

so lots of ideas , you just have to decide what is best. good luck

Here is a link that might be useful: mosaic basics store