Hi Rachel,
First thing you should know about setting faceted stones in resin is that they will no longer sparkle like they do in the open air. I’m sure there is a physics equation that can explain it, but the best way I can tell you is that it deals with the light passing through the resin and it not refracting off the facets like it does in the air. Perhaps someone smarter than me can give a better explanation.
If you are okay with that and want to try, I’m guessing you want to cast the diamond such that it looks suspended in the resin. To do that, you will need to pour in (at least) two layers. If I were going to try, and I wanted a gemstone suspended halfway in the casting, I would find a way to hang the diamond into the mold, then cast it half full. Once the resin has at least partially cured to the point of being able to hold the stone by itself, I would cast the next part.
This other discussion on the forum may help you as well: https://www.resinobsession.com/forums/topic/making-a-sphere-with-a-suspended-gemstone
Good luck!
