Osmium is a chemical element. It has
the symbol Os and it has an atomic number of 76. The
atomic weight is 190.23. 
Things you might not know about
Osmium:
- The stylus (needle) in early phonograph
designs was also made of osmium, especially for 78-rpm records,
until sapphire and synthetic diamond replaced the metal in later
designs for 45-rpm and 33-rpm long-playing records.
- Osmium tetroxide has been used in fingerprint
detection.
- Osmium alloys such as osmiridium are very
hard and, along with other platinum group metals, are used in the
tips of fountain pens, instrument pivots, and electrical contacts,
as they can resist wear from frequent operation.
See GC2XAY2 - Hydrogen, for an explanation
about this series of caches.