I'm on attempt 67 and 68 (had dragons breeding in both upgraded caves since it came out). I've not had a single Diamond Dragon and it's very frustrating! I've had 9 Motley dragons, and 3 Platinum, but no Diamond!
The last two gem dragons I managed to get in the LAST 24 hours, and using a crapload of gems. I'm almost to the point of believing that you need to just buy the gemstone dragons if they are on sale and save yourself gems and a crapload of time!