The Hope Diamond: Solving the Mystery

by Jasmine Catlow, MChem Chemistry. Originally published in issue nine of Resonance.

The Hope DiamondIn the mid-17th century, an exceedingly rare diamond, coveted for its unusual blue lustre and its expansive size, came into the possession of King Louis XIV of France. Known as the French Blue, it displayed a characteristic heart shape cut. However, during the French Revolution, the contents of the royal vault were ransacked, and though almost all the other jewels were reclaimed, the French Blue was never found.

Decades later, in London, another blue diamond surfaced - purchased by Henry Hope, with whom the name and curse of the Hope Diamond originate. Said to bring disaster and misfortune upon those in its custody, Hope’s family went bankrupt and the diamond had to be sold for survival. It changed possession several times, with many owners supposedly falling foul to terrible tragedies, before a lady named Evalyn Walsh bought it. Attracted by the misfortune it was said to bestow upon others, she wore it as a lucky charm. That was, until her son died in a car crash, her daughter committed suicide, and her husband was deemed insane. After dying alone, her entire jewellery collection was bought, and the Hope Diamond quickly donated to avoid the curse’s wrath. Today, the diamond remains on display to the public, in the US national gem collection.

Given that the French Blue, of acclaimed character, never resurfaced, it was widely thought to have been recut into a less distinctive shape. Add to this, the mystery surrounding the Hope Diamond’s appearance, and it isn’t too much of a step to ask if the cursed Hope Diamond was cut from the stolen French Blue.

Evalyn WalshTo answer this question, a team of scientists and gem-experts recreated a 3D model of the French Blue from a to-scale engraving and a lead replica made during the reign of King Louis XIV. Data for the Hope diamond was also collected and the two computational image sets were compared. At the perfect angle, the Hope diamond was found to fit perfectly inside the structure of the French Blue; confirming that the former was cut from the latter. However, the question remained; where is the rest of the French Blue? Fortunately, modern analytical techniques may shed some light on this.

The exact composition of authentic blue diamonds is vague, as not only are they rare but it’s difficult to analyse them without destroying them. Adding to this frustration, any impurities are so low in concentration that they can’t be detected by infrared spectroscopy, so special techniques such as secondary ion mass spectrometry (SIMS) or phosphorescence spectroscopy must be employed.

When examining the two types of blue diamond - type 1a and type 2b, 1a diamonds are more common, and contain significant nitrogen impurities, as well as hydrogen. It is the presence of hydrogen that gives them a secondary hue (e.g. blue-green). In complete contrast, 2b diamonds contain almost no nitrogen impurities and, instead of hydrogen, possess a quantifiable amount of boron. This means that the rarer type 2b blue diamonds normally exhibit a darker hue, and are the only semiconducting diamonds. The Hope Diamond falls into the type 2b category.

Blue diamonds phosphoresce under UV light - normally a blue/green colour, but the Hope Diamond phosphoresces red. This red phosphorescence was originally thought to be rare, but has been found to be characteristic of all type 2b blue diamonds. The reason the Hope Diamond phosphoresces red is to do with its composition, and is thought to be a reason why it was perpetuated as being cursed. Most blue diamonds exhibit both a red and a blue/green emission, but the red colour is normally masked by the blue/green emission. Diamonds that exhibit both these emissions are higher in boron, with low nitrogen traces; type 2b. Type 1a diamonds exhibit an absence of these two emission peaks. The half lives and intensities of diamond phosphorescence are due to a difference in boron to nitrogen concentrations.

The phosphorescence of blue diamonds provides a unique photophysical identifier, so if the Hope Diamond was ever stolen again it would be easily traceable, even if it was recut. Perhaps more interestingly, other known blue diamonds and those which emerge can now be analysed in a search for the missing pieces of the once great French Blue - solving this centuries-old mystery.

Jablonski DiagramPhosphorescence: A Quick Guide

Phosphorescence is a photophysical process in which an electron in the excited state of an atom or molecule undergoes intersystem crossing, and then decays back to its ground state by emission of a photon – hence the associated colour change! It differs from fluorescence as it is typically spin-forbidden, thus it has a longer lifetime.

As shown in the Jablonksi diagram on the right, intersystem crossing has an associated energy loss. Thus, phosphorescence emissions are redshifted compared to fluorescence, giving the red emission of the Hope diamond!

Diagram by Smokefoot, Wikipedia

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