Iridium and similarly hard metals are difficult to machine ...
One issue with Zirconium for instance, is that as it gets hot, it forms a very hard oxide layer. When cutting Zirconium, slow and cool makes it a less difficult endeavor than just going at it as hard and fast as possible. I could cut Zr with a hacksaw, but my cutting mill could not bit into it more than a few millimeters due to that.