Quote from the wikipedia article on iridium:

Iridium's modulus of elasticity is the second-highest among the metals, only being surpassed by osmium.[5] This, together with a high shear modulus and a very low figure for Poisson's ratio (the relationship of longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty.

Why do these properties make it difficult to machine apart from their effects on hardness?

The Vickers hardness of iridium is extremely high for a metal (1760–2200 MPa), but bulk iridium is still easily scratched by harder objects(like quartz, I have seen this first hand).

Is "resistance to deformation" not the same thing as resistance to scratching?

If it can be scratched by a harder element, what makes machining such a monumental task?

Could you not just use a diamond bit etc?

What makes it more difficult to machine than any other hard material?