need to try this ... 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. I used 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.
Bake marbles at 325-350 degrees for 20 minutes (for more cracking-look, bake at 500 degrees for 30 minutes). Remove and carefully drop into pan with ice and water. Glue end caps to finished marble (w/starter rings) to create bracelets, earrings, pendants, ornaments