Salty you can still get regular supernovium by putting 5 different types of ingots on the display case. You just can't get the direct large supernovium anymore from placing multiple (irridium/platinum/mummotomium).
As far as the skull cut goes, as long as you have 5 different cuts on the table you will get a skull cut, however from my testing it seems to be random as to what gem type becomes a skull cut. So the only way to guarantee a skull cut is to have all 8 slots filled with the same type of gem. I had a case where I had 7 gems of one type, each a different cut, and put in one emerald cut emerald and got a skull cut emerald out of it.
