Historical clue formulations

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