Why is osmium difficult to use despite its hardness and high melting point?

Osmium is difficult to work with primarily because it is extremely hard and brittle, even at high temperatures. Furthermore, it possesses the highest melting point among the platinum metals, which makes the processes of fusing and casting the metal particularly challenging.1

Beyond its physical properties, osmium presents significant handling risks. It is the most rapidly attacked by air among the platinum metals; even at room temperature, the powdered metal releases a poisonous, volatile vapor known as osmium tetroxide (OsO₄). Because of these difficulties, osmium is rarely used in its pure form and has largely been replaced by ruthenium in alloy applications.12