When I first wanted to sculpt gemstone, I tried to checked on the net for tutorial but unfortunately I can't find any. So I just have to figure it out myself. After 2 times trying, I've done. And here I like to share to everybody who wants to sculpt it.
Tools: sculpting tool or needle or pin or pen knife. For me I use this sculpting tool. Its my favourite....
1) Roll a small mixed putty to the shape you want either oval or round and also the size. Then press it on an unused plastic sprue or you can directly press on the model's surface. Use you finger to polish it till it looks shinny. And then wait.....
2) Wait till the putty get harder. Use you sculpting tool gently touch it. As long as you find it hard, non-sticky, and creamy, it is ready! Use the flat side of your sculpting tool to press the rim (as shown above with red line).
3) Now the tricky part. Use the flat side of sculpting tool again, gently press the rim and pull sideward motion to another end.
4) You will find your gemstone pop out from the rim. Now again use the flat side of sculpting tool to smooth the edge of gemstone. And refine the shape of the rim if you find it not the way you want. If you sculpt on a plastic sprue, you can slice it out with your pen knife and then glue it to your model.
Well, there goes your gemstone. You just have to practice more to get it done nicely. If you have any questions, just post a comment here and I will answer it.