D. L. Windt, W. C. Cash, M. Scott, P. Arendt, B. Newnam, R. F. Fisher, A. B. Swartzlander. Optical constants for thin films of Ti, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Ir, Os, Pt, and Au from 24 Å to 1216 Å, Appl. Opt.27, 246-278 (1988)
Osmium (Os) is a dense, bluish-white transition metal that belongs to the platinum group in the periodic table. It is the densest naturally occurring element, with a density of around 22.59 g/cm³. Osmium has a very high melting point and is one of the least reactive elements. It is mostly used in alloys with other platinum group metals, enhancing hardness and durability. Due to its hardness, it's often used for the tips of instrument pivots, as well as electrical contacts. In the realm of optics, osmium's primary use has been as osmium tetroxide, which serves as a staining agent for biological samples in electron microscopy. However, osmium itself, being opaque, does not have notable optical applications. Safety note: Osmium tetroxide is highly toxic and should be handled with extreme care.