The mysterious affair of the 'Brunswick Blue' diamond has long continued to interest gem historians. It suggests the title of a work of fiction and, indeed, the central character in the chain of events appears to a considerable extent more fictitious than real.
The diamond's owner, Charles 11, Duke of Brunswick, was an unpopular ruler who, after an unfortunate reign, was driven from his land by a popular rising in 1830. He was also an ardent collector of jewels - and an eccentric. He owned a collection said to have been valued at £500,000, which he kept in his house in Paris. Such was his passion for his prized jewels that he would not spend a single night away from his residence, which had been built more from the point of security than comfort. It was surrounded by a high thick wall on top of which was a 'chevaux-de-frise', so arranged that when a hand was laid upon one of the spikes, a bell immediately rang.
The diamonds were kept in a safe let into the wall and Duke's bed was situated in front of it. If an attempt had been made to force open the safe, four guns would have been discharged, thereby hopefully killing the burglar on the spot, and connected with the discharge of the guns was a mechanism which would ring alarm bells in every room of the house to arouse the household.
The bedroom had only one small window; the bolt and lock of the door were made of the stoutest iron and could only be opened by someone who knew the secret. Finally, a case containing twelve loaded revolvers stood by the side of the bed.
It is reassuring to know that diamonds were as highly prized in the last century as they are today, but doubtless the Duke had in mind the events surrounding the theft of the French Crown Jewels from the Garde Meuble, in September 1792.
After the Duke of Brunswick's death in 1873 some of his jewels were put up for sale in Geneva in the following year. One of them the 30-carat 'Brunswick Yellow' diamond, was bought by Tiffany's. Among the other gems was at least one diamond of a rare dark blue colour, the weight of which remains uncertain to this day.
Unwittingly, the noted London jeweller and gemologist, Edwin Streeter, is partly to blame for this uncertainty, for in his two books he has written contradictory accounts of the diamond's weight. When he examined the stone he concluded that is must have been cut from the 'French Blue' diamond, weighing 67.5 (old) carats, which had been among the Crown Jewels since the time of Louis XIV. It has always been assumed that the 'Hope' diamond, weighing 45.52 metric carats, (44.34 old carats), constitutes the major portion of this legendary stone and that some time after its theft from the Garde Meuble it had been recut to avoid detection. In the 1882 edition of his celebrated work 'The Great Diamonds of the World' Streeter considered that after it had been cleaved the 'Hope' became the larger piece while an irregular triangular-shaped piece would have remained. (If the 'French Blue' had been fashioned to produce the 'Hope' diamond around the year 1800 it would, for technical reasons, have been cleaved, not sawn.)
Streeter thought that if a drop shape of the same colour as the 'Hope' weighing from 12 to 13 (old) carats with its base corresponding to the straight side of the 'Hope', were to be found then there was enough presumptive evidence to suggest that it must have formed part of the 'French Blue' diamond. He wrote as follows:
"Such a stone did actually come into the market in April 1874. It was purchased in Geneva at the sale of the late Duke of Brunswick's jewels. The purchaser put the stone for a short time into my hands and I examined it juxaposition with the 'Hope' diamond. It is identical in colour and quality.
I know not how to avoid the conclusion that the Duke of Brunswick's 'Blue Drop' diamond once formed the triangular salient gibbosity which formerly appears to have characterized the stone now known as the 'Hope' brilliant. Besides the 'Hope' and Brunswick diamonds, there are only three diamonds known in Europe that can justly be termed 'blue', and these all differ from the 'Hope', and from each other in colour".
Yet in the sixth edition of 'Precious Stones and Gems' published in 1898 Streeter's account differed considerably. Here he stated that the original 'French Blue' had been cut into three, not two, pieces. The 'Hope' diamond remained the larger portion while the weight of the Duke of Brunswick's diamond was reduced to an estimated weight of between 6 to 7 (old) carats, to allow for the existence of a third piece.
Streeter stated that he bought this third diamond, known as the 'Pirie', for £300 in Paris; it weighed 1 (old) carat and was identical in colour to the 'Hope'.
It is extraordinary, to say the least, that such an expert could have written two such varying accounts of these stones. However, the earlier theory about the weight of the Duke of Brunswick's diamond has been clearly disproved by Albert Monnickendam in the book 'The Magic of Diamonds'. In Mr Monnickendam's opinion it would have been technically impossible to have cut a drop-shape weighing from 12 to 13 carats if the principal piece, i.e., the 'Hope', cut from the 'French Blue' weighed as much as 44 or 45 carats. In his view, which is one currently agreed upon by modern authorities, the pear-shaped diamond could not have weighed more than about 10 carats.
We are left, therefore, with Streeter's second account stating that the 'Brunswick Blue' diamond weighed between 6 and 7 carats. If such a stone comes to light the owner will possess not only a gem of a colour still rare today, in spite of further discoveries in southern Africa, but one of exceptional historical interest. Ultimately scientific examination may produce the answer to the mystery. When the 'Hope' diamond came to South Africa in 1965 to be put on exhibition, the opportunity was taken to make a thorough investigation of its physical properties and it was found to be the only blue diamond known to phosphoresce following exposure to ultraviolet light.