Osmium is a chemical element. It is element number 76 on the periodic table. Its symbol is Os.
There is not much osmium on earth. There is actually such a small amount that osmium is very expensive.
Osmium is a metal and it does not react very much, and such a metal is known as a "noble metal".
Osmium is the densest (heaviest for the same amount) element on the periodic table.
It was discovered in 1804 by Smithson Tennant in London, England. He found it together with Iridium in a bit of Platinum, which he had dissolved in Aqua regia.
Osmium has a blue-grey hue and is the densest stable element; it is about twice as dense as lead and slightly denser than iridium. Osmium is a hard but brittle metal that stays shiny even at high temperatures. It has a very low squeezability. Accordingly, its size module is extremely high, reported between 395 and 462 GPa, which is comparable to that of diamond (443 GPa). The hardness of osmium is moderately high at 4 GPa. Because of its hardness, brittleness, low vapour pressure, and very high melting point, solid osmium is difficult to machine, form, or work.