These look cool! I found them on pinterest and looked into it. Here's what I found from craftster.org
"These are marbles that I fried, well actually baked causing them to crack on the inside but still remain intact on the outside, so I glued end caps to them with starter rings and created some beautiful pendants. I was going to drill through them and make beads, but as we make mistakes we learn. I learned that I should probably drill the marbles before I crack them, otherwise they fall apart when trying to drill through cracked glass."
"It's really quite easy the best method I found for getting the most cracks is to 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."