It's not normally. Polished Split Cut is for geode and septarian nodules only. Spire Cut is for tiberium only. The way that myself and others have gotten it in different gems was through
transfiguration. It can sometimes take a long time , a lot of gems, or you could save and exit without saving a lot.
I tend to only try this in challenge files where exiting without saving is cheating so I go through a lot of gems.