Natural History Collections in the Science of the 21st Century. Группа авторов

to the Louis XIV blue diamond was based on the concept of critical angle (θ₂), which is related to the angle of total reflection itself as a function of the diamond’s refractive index, worth 2.42 on average (i.e. θ₂ ~ 26°, Figure 3.4(a)). The light path (ray 1 in Figure 3.4(b)) passed through the diamond given its angle of incidence (θ₁) zero with respect to the perpendicular angle of the diamond table. It passed through the diamond/gold diopter and returned the image of the underlying gold setting. The path (radius 2 in Figure 3.4(b)) corresponds to the incidence of light on the slightly off-center diamond (θ₁). Again, the angle of incidence was less than the critical angle and the light was still reflected by the gold. As for the third path (ray 3 in Figure 3.4(b)), it corresponded to the case where the light arrived on the facets of the pavilion with an angle greater than the critical angle: the light was then reflected in the diamond which increased its absorption and, consequently, the color perceived by the observer in return. Clearly, Louis XIV’s gem cutter had mastered, knowingly or not, the founding concepts of linear optics published by Descartes (1596–1650) in La Dioptrique (Descartes 1637). The computer simulation of the Louis XIV blue diamond gave a diamond with blue hues centered on a sapphire blue. In the 17th century, the term “violet” was used to designate both dark blue and the current violet. As far as the Louis XIV diamond is concerned, it is the “sapphire blue variant” of “violet” that is the correct color for this diamond.

The 1691 Crown Diamond Inventory (AN, O¹ 3360) tells us that the diamond was mounted on a “baton d’or émaillé par derrière” (gold stick enameled from behind). A second computer simulation (Figure 3.4(c)) of this diamond set in this manner was calculated following these archival indications. The result showed a central seven-pointed gold figure set in a deep blue background and studded with white spot chips. Clearly, this star was the emblem of the Sun King, Louis XIV, and, more generally, of the monarchical France of the time, whose symbolic colors were celestial Marian blue and solar gold (Farges 2014). The seven golden facets located around the central facet (culet) symbolize the seven planets then considered (Mercury, Venus, Earth, Mars, Jupiter and Saturn, plus the Moon). Depending on the inclination of the diamond, these facets lit up or went out while the culet remained “always lit with gold”. We postulate that this corolla of seven facets represented an allegory of the graces and disgraces of the king toward the heavens, which were subject to his good will and in precession around the solar star. This “royal sun” shone in a blue background studded with stars representing the universe. These parables can be found in the allegories contemporary to the making of this decidedly extraordinary diamond, such as those of the Ballet de Flore (1669) where the king danced before the Court in the presence of the four white horses of Apollo’s quadriga: the libretto by Isaac de Bensérade (1612–1691) multiplied the allusions to the benefits of the sun on the Universe while the king was returning victorious from his first European conflict, the War of Devolution (1667–1668).

      This heliocentric vision contrasted with the Platonic dogmas then recognized by the Church in which the Sun was supposed to orbit the Earth. It was not until 1755 that the Vatican admitted this “modernist” version of the mechanics of the solar system. Clearly, Louis XIV did not wait for the papacy: Descartes had already been published in his post-humous opus The World (originally dated 1633; Descartes 1664) where he took up the cause of Galileo and his assertions on heliocentrism. Through his “modernist” vision, Louis XIV asserted his Gallicanism above all against the ultramontanes (and other “Vatican-ultras” in France like the Compagnie du Saint-Sacrement (Company of the Blessed Sacrament) who were more subservient to the Vatican than to the power of the young sovereign, which the latter wanted to be absolute.

      As for the Grand Saphir (Figure 3.2), it was also set in gold as the royal inventories indicate. We have been able to undertake a gemological study since the original gem has been preserved today: it constitutes, along with the Hortensia diamond of the Louvre Museum, the only two remains of the fabulous gem acquisitions by Louis XIV that have been kept intact to date. UV-vis-NIR spectroscopic analysis (Farges et al. 2015) of this gem shows that the sapphire originated in Sri Lanka. We can even see growth zones and inclusions that allow us to conclude that this sapphire has not been heated to accentuate its blue color. The zonations show that the original crystal was much larger, but these extra volumes must have been less transparent and were removed by recutting.

      In the end, it is astonishing that the two blue gems of Louis XIV ended up in the Muséum national d’Histoire naturelle: one intact and the other in the form of a lead cast, the original of which has definitely disappeared. To a certain extent, this long-ignored lead cast has become the “original” and its precise study shows that its creator – still unknown at this stage – wanted to keep the trace of exceptional expertise by making this historical replica. My very recent observation (end of 2018) of the surface of this lead cast would seem to indicate that the date “1806” is inscribed on it. This type of lead casting is usually done prior to a sale or re-cut so that the gem cutter can keep track of his excellence for future customers. If this hypothesis is true, this information would indicate the date of the recutting of the Louis XIV blue diamond, which corresponds exactly to the time window between the theft of the French diamond in Paris and the first appearance in London (1792–1812) of what would later be named the Hope diamond, now kept at the Smithsonian.

      Figure 3.4. Optical principles of a generic faceted diamond explaining definition and measurement. © F. Farges/MNHN

      COMMENT ON FIGURE 3.4.– (a) Measurement of the critical angle, θ2, for an air (1)/gem (2) interface (also called a diopter). For a light incidence (θ1) smaller than this angle, the diamond will behave like a transparent diopter. Otherwise, the facets of the gem will behave like mirrors: the light will be more absorbed and the color will be perceived as more intense (darker); (b) optical principles for three typical light paths; (c) computer simulation of the Louis XIV blue diamond on the basis of the MNHN inv. 50.165 cast and the color of the Hope diamond and then placed in a model of a setting according to the 1691 inventory.

3.4. Epilogue: toward a renaissance...

To free ourselves from the virtual, we have reconstructed the “almost real” diamond. The challenge is potentially expensive: if this mineral had actually existed, the price would have been stratospheric. However, another path was considered: cubic zirconia, ZrO₂, is a crystallized zirconium oxide that has optical properties (refractive index and optical dispersion) close enough to those of a diamond without costing a fortune. But no cubic zirconia has the “cold blue” color so typical of blue diamonds. On the contrary, they show “warm” blues matted with purplish red, which gives us the wrong idea of Louis XIV’s blue diamond. We then opted for a “physical” solution. Indeed, it is possible to give the illusion of a color by sputtering, that is, depositing metallic particles “nano-pulverized” at high temperature and under ultra-high vacuum on a substrate of colorless zirconia. The color – obtained by light scattering by this thin layer of nanoparticles – is adjustable according to the operating conditions (ultra-high vacuum, temperature, nature of the metal to be deposited, etc.). After 2 years of testing, a colorless zirconia cut with the exact shape and faceting of Louis XIV’s blue diamond was “nano-coated” on its surface: it almost perfectly reproduces the theoretical simulation of the original blue diamond (Escobar 2017). Moreover, this replica will soon be set in gold so that we can see this magnificent diamond again as it once appeared in the eyes of the Sun King: a political instrument in the colors of a brilliant monarch. As amazing as it stands, no gem cutter – even one with modern, digitally enhanced skills – would have dared to imagine such a level of sophistication.

3.5. References

      Bapst, G. (1889). Histoire des Joyaux de la Couronne de France. Hachette, Paris.

      Descartes, R. (1633). Traité du monde