A member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high melting point, solid iridium is difficult to machine, form, or work; thus powder metallurgy is commonly employed instead.It is the only metal to maintain good mechanical properties in air at temperatures above 1,600 °C (2,910 °F).It has the 10th highest boiling point among all elements and becomes a superconductor at temperatures below 0.14 K.
Iridium's modulus of elasticity is the second-highest among the metals, only being surpassed by osmium.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. Despite these limitations and iridium's high cost, a number of applications have developed where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology.
The measured density of iridium is only slightly lower (by about 0.12%) than that of osmium, the densest metal known.Some ambiguity occurred regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately,but, with increased accuracy in factors used for calculating density X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium.