Baked marbles! bake the marbles on a cookie sheet at about 325 - 350 for 20 minutes, as soon as you take them out of the oven put them in a metal bowl or sink of cold water and ice. Use plain marbles usually found in the floral department of crafting stores. Any colored marble will work as long as it is translucent, the marbles that are fully colored like toy marbles are kind of useless because the cracks won't show up. This also works with glass stones.