The History and Chemistry of HOPE

Diamonds are rare; blue diamonds are even rarer, constituting   less than 0.2% of naturally occurring  diamonds.   Experts say that  there are only a handful of notable blue diamonds in the world. And  Hope  tops that list.  This  most unique, priceless,  brilliant  deep blue diamond  is  currently  held  most securely  at the  Smithsonian Museum, Washington DC.  More than 350 years ago, John Baptiste Tavernier,  a  French trader, traveler, fortune hunter held it in his palm and  exclaimed at the  beautiful violet,  but the usual color qualifier has always been Blue.   

  

Hope Diamond with lighting  (Courtesy Wiki)

The birthplace  of Hope  diamond has been identified to be  the mines of Golconda, India. Sometime during the   seventeenth century (some say1640), Tavernier reportedly  appropriated  a large, 112.5 carat  uncut blue crystal from a local. Tavernier  later traded it  for a price and position  to the French emperor  Louis XIV.  The king had it  chiselled and set  to a triangular shape of 67.2ct. It acquired the name  French Blue and  remained among the French Crown jewels until  revolution broke out in the  1790's.

 Tavernier's original sketch of French Blue Courtsey: wiki 

 

During the revengeful  loot and plunder of the royal household by the enraged  masses, French Blue  disappeared along with other crown jewels.  Later  during the early part of nineteenth century a brilliant blue stone surfaced in the London jewellery market. John Francillon, an English jeweller had his suspicions. There were speculations that this was indeed the French blue. However  whosoever owned it in the interim period had it reduced  in size  to 45.52 ct  and reshaped it - perhaps to escape  knowledgeable eyes-  before floating it in the market.  The blue stone eventually became a part of  the English crown jewels collection.  In 1830, Henry Philip Hope, a wealthy banker  purchased  it from the  English King George IV, purportedly as part of a debt settlement.   The gem acquired a  new, permanent  identity The Hope Diamond, a name that didn't change any further with changing ownership.  In 1958  Hope Diamond was donated   to the Smithsonian Institution, Washington, DC, by the then owner.

So much for the history of Hope. Its  chemistry  (and also of all natural blue diamonds) is  equally interesting.  Chemists were fascinated by the brilliant blue and embarked on the job of identifying the cause. In 1971  it was established categorically that traces of boron   imparted the  blue colour and not aluminium as was believed till then.  It was also seen that  boron would  impart blue colour, only in the absence of other impurities. Hope diamond  underwent a systematic, rigorous  scientific grading by the Gemological  Institute of America (GIA)  in 1988.    Non-destructive spectroscopic techniques such as Infrared, Ultraviolet and  Pulsed luminescence were used to estimate the boron content.  Hope recorded  a value of 0.6 ppm boron (parts per million). Diamonds are in general insulators, but natural blue diamonds are p-type semi-conductors.  Because trivalent boron in  a  lattice work of tetravalent carbon leaves vacant slots, i.e. holes which facilitate electron jumping.    

While chemistry was thus explained, geochemistry still remained elusive for a long time. Because  boron is available only in the continental and oceanic crust which run to   an average  depth  of roughly 65 kilometres only.  Whereas diamonds are known to be formed at a depth of 100-250 kilometres, in the upper mantle. Several studies alluded to an even deeper zone, a depth of 660 kilometres, for blue diamond formation.  So how did boron travel so far  down?  Perhaps we have an answer now. "Geological pathway for recycling of Earth's surface materials  into the mantle are both driven and obscured by palte tectonics" contend  Smith et al  in the August 2nd issue of Nature Magazine.  When continental crust slides over the  oceanic crust, lithosphere rich in rocks and minerals  gets  pushed down through serpentine pathways  into the lower mantle. 

Geological process of subduction: Courtsey wikipedia                                                                                                                                                    Author: K. D. Schroeder Subduction-en.svg from Wikimedia Commons                                                                                                                                  License:Creative Commons Attribution-ShareAlike 4.0

References:

1. History of the Hope Diamond, Smithsonian Institution

2. Grading the Hope Diamond

3. Natural color Blue, Gray and Violet diamonds-Allure of the Deep

4. Blue boron-bearing diamonds from Earth's lower mantle : Smith et al Nature 560, 84-`87, 2018