As mentioned gum and elm are much tougher than hickory and there are others that get stringy and twisted sometimes. White oak can be stringy if the tree has grown in the open, instead of the middle of the woods. I often find more difficulty when splitting trees from fence rows or open fields. A good sharp hatchet helps when the strings are the problem. I also employ the duel pry bar method, using two 36" crow bars, inserted in the end of the split, facing opposite directions while the wood is still standing on end and has already been opened an inch or more with a wedge or maul. Pry simultaneously with each bar in opposite directions (the top of one bar is pushed away from you and the top of the other bar is pulled to you at the same time). Different size bars can be used. I simply like a matched set of 36" and have 2 dedicated to firewood.