Osmium is the densest known metal element, with a density of 22.48g/cm3, a melting point of 3045°C, and a boiling point of over 5027°C. Hexagonal dense lattice. Gray-blue metal, hard and brittle, can easily become powder when pounded in an iron mortar. Osmium powder is blue-black, and osmium metal powder can spontaneously combust. Osmium vapor is highly toxic and can strongly irritate the mucous membranes of the eyes, causing blindness in severe cases.
Osmium metal is very stable in air, and powdered osmium is easily oxidized. Concentrated nitric acid, concentrated sulfuric acid, and sodium hypochlorite solution can all make it oxidized. It is easy to react with oxygen at room temperature to generate osmium oxide (OsO2), and when heated, it can generate volatile and highly toxic osmium tetroxide (OsO4).

Reflective tin white with light blue. Reflectance: 67.5 (green light), 66 (orange light), 67 (red light). The double reflection of the plane parallel to the c-axis is clear. Heterogeneous weak to medium strong, pale pink, red, bronze and gray to dark blue.