QUARTZ
| Date | Clue |
|---|---|
| 2017-08-20 | The crystal in some crystal balls |
| 2014-10-05 | Amethyst or citrine |
| 2011-04-09 | What a crystal ball gazer sees |
| 2007-09-07 | Tiger's-eye, essentially |
| 2006-06-13 | 7 on the Mohs scale |
| 2001-03-02 | Hard rock |
| 1994-02-10 | Seven on the Mohs' scale |