OSMIUM

Densest of all naturally occurring metallic elements, Os (6)

Solution has appeared in 4 crosswords (1 across and 3 down):

• Quick crossword No 16,679 — 2023-10-19 23:00:00
  4D: Heaviest known metal – I'm sumo (anag.) (6)
• Quick crossword No 16,061 — 2021-10-27 23:00:44
  16D: Heaviest known metal, Os (6)
• Quick crossword No 15,783 — 2020-12-07 00:00:23
  13A: Metallic element used in alloys, Os (6)
• Quick crossword No 15,290 — 2019-05-09 23:00:14
  19D: Densest of all naturally occurring metallic elements, Os (6)