I'm betting it was the size. I have used 4mm before and had cracking. Try 1.5 or 2mm. They do look really tiny but the sparkle is still big
Cast in place means you can heat them enough to cast them in metal. No backing or any special look, just that they are able to withstand heat. I have never had a cz crack, even the big suckers I tried (4mm that is, lol).
I heat the snot out of my cz's so you don't need to baby them. PM me your address and I'll send you off some that are cast in place. Then you will know for sure they are the right ones.
Do you know how to glue them to the end of a mandrel so you can get them in the right spot easily?