Pam already noted that it is really a matter of chance and no exact formula. In my experience, you need 5 different cuts and same gem type to get the skull cut of that gem for shore. Since with tiberium you can't get them cut in more than 2, you need to rely on different and uncertain formula - 5 different cuts (2 tiberium and 3 from other gems) and you still have chance to have one of those gems in final result. Technically you can place 5 tiberiums - one spire other heart cut, and 3 emeralds in 3 cheapest cuts and hope for the tiberium skull, not the emerald skull (since it is still 5:3 chances on tiberium skull to appear).
To make the process faster with less loss, you can save the game after placing gems but before transfigurating them, and exit without save back to main menu if you don't like the result. I used the small not developed world for that project as it saved me the saving and loading time. If you want to do it in regular game then it there is a reasonable chance that you'll lose a lot of tiberiums to get that skull. Spire cuts of gems are giving the highest value of the gem, but only few gems are valuable enough to be worth the investment of several tiberiums involved in transfiguration. Others are just nice to have in the collection.
Also, to save money on spire cut, you can sent and cut all tiberiums in heart shape, then place one or two on the ground and keep the others in inventory, those heart shape should turn into spire and you'll get spire cut tiberium for 5.500$ less per gem.
If you have Pets, get a dog and let him find gems too, it can speed up the process. With board breaker, I find that Lapis Lazuli and Fluorite are rarest gems that spawns in my game and that there are few tiberiums in each session at the board breaker, but I just might be lucky there.
Good luck in getting that skull
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