OPAL

Form of hydrated silica, often used as a gemstone (4)

Solution has appeared in 3 crosswords (2 across and 1 down):

• Quick crossword No 16,537 — 2023-05-07 23:00:33
  16A: Translucent mineral used as a gemstone (4)
• Quick crossword No 15,939 — 2021-06-07 23:00:04
  12D: Semi-transparent gemstone, (4)
• Quick crossword No 15,487 — 2019-12-27 00:00:24
  11A: Form of hydrated silica, often used as a gemstone (4)