Last weekend I tried with Sakrete (which contains stones in a mix with cement) and the results did not come out well. Here are pics from before and after attempting to remove from the mold. The left is just a giant bowl I bought at Ikea. On the right, I bought a light fixture glass bowl at Home D.
Look how the bowl shape just crumbled apart! I was able to break it apart in my hands into the pic on the right. I didn't try to release the ball, but I assume it would have come out just as crumbly as the bowl. So I went to Home Depot and bought some RapidSet plain cement instead, and I hope it will turn out better. The verbiage on the box claims it's good for casting.
I also created some small objects for casting. And then I made molds from those small objects and from several acorns I found: