Personally I feel it is just random on the combines. I have my pottery trophy at journeyman skill, along with journeyman skill trophy for jewel craft. I think you can be at any level and randomly miss, (once trivia led) at what you are striving to get. So don't worry it will come. And remember it is just a game. O and I managed to get my mounted gems as follows:
1. Blue diamond: first try
2. Jacinth: first try
3. Diamond: first try
4. Black Sapphire: first try
5. unfired seal: first try
6. fired seal: first try
So I do actually consider myself lucky as far as getting my mounted jems on the first try. Considering the price of the vellium bars lol. I just bought mine.