Hi Lorrie,
To suspend your sandstone in the resin, you will need to pour in layers. Pour your first layer of resin and wait for it to partially to fully cure before adding the sandstone. Then, pour your next layer. You can see what I’m talking about in this article:
https://resinobsession.com/resin-tutorials/button-bangle-resin-bracelet-tutorial/
To make a hole for a hook, you can drill the resin once it has fully cured. I show a way to do that here:
https://youtu.be/M7Ippav162g
Since you are casting in a mold, I recommend the Resin Obsession super clear resin. It cures crystal clear and is meant for molds. If you are careful in mixing, bubbles are almost never a problem. We have it in several sizes in our store here:
https://shop.resinobsession.com/collections/resin/resin-obsession