Historical clue formulations

• Like many engagement stones — 2021-02-06
• About 2% of the Hope Diamond — 2016-06-11
• Common diamond measure — 2014-12-31
• 200 milligrams — 2008-09-19
• About three grains — 2007-06-01
• Like some engagement rings — 2002-06-01
• Like some diamonds, sizewise — 1997-07-07