QUARTZ

Hard mineral consisting of silicon dioxide in crystal form (6)

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

• Quick crossword No 16,504 — 2023-03-29 23:00:47
  17A: Hard mineral composed of silicon dioxide (6)
• Quick crossword No 16,400 — 2022-11-29 00:00:11
  3D: Colourless glass made of almost pure silica (6)
• Quick crossword No 15,914 — 2021-05-09 23:00:31
  13A: Hard mineral consisting of silicon dioxide in crystal form (6)