French Blue
Specifications
| Weight | 68.9 carats* |
|---|---|
| Dimensions | 31.00 x 24.81 x 12.78 mm |
| Color | Blue |
| Weight of Rough | Cut from Tavernier Blue |
| Origin | Tavernier Blue |
| Date Found | Cut in 1671 |
| Current Location | Cut into the Hope in the early 1800’s |
Details
The French Blue: Dimensions, Debate & a Diamond Lost to History
Brisson’s Measurements vs. Golden Fleece Drawing
Morel (1986), citing Brisson’s Imprimerie Royale (Paris, 1787, pp. 68–69), offers one of the most oft-quoted technical descriptions: dimensions of 31.00 × 24.81 × 12.78 mm, with a weight of 69.00 metric carats. These figures are considered one of the more authoritative references for the stone.
Drawing of French Blue from Fleece
Stretched Model
However, the historical drawing of the Golden Fleece—the grand jewel in which the French Blue was once mounted—presents a stone with dimensions of about 30.96 × 26.19 mm, noticeably narrower in one axis and wider in another. This discrepancy raises an important question: which source is more reliable—the measured record or the artistic rendering?
When constructing a replica, modellers began with the Golden Fleece line drawing and gradually “stretched” it to match Brisson’s measured values. After several iterative adjustments, the working template settled at 30.96 × 24.87 mm, which represents only about a 0.2% error from Brisson’s 31 × 24.81 mm. This compromise reflects a belief that the Golden Fleece drawing, though detailed, may suffer from proportional distortions introduced through reproduction and printing—whereas Brisson’s values likely stemmed from careful measurement tools of the era.
This methodological tension—between visual historic art and recorded measurement—is a common theme when reconstructing rare historical diamonds from fragmentary data.
The Break Facets Debate
One particularly thorny technical issue revolves around the break facets—those small facets along the girdle (the junction of crown and pavilion). Some scholars and cutters argue that these facets were vertically split—meaning what appears as a single facet in a drawing is actually two facets in plane, vertically bisected. This hypothesis is motivated by an attempt to align symmetry with the pavilion’s own split facets and to maintain continuity in facet layout.
The counterargument, however, points to the detail (or lack thereof) in the Golden Fleece drawing. The artist would have had no trouble delineating split facets by simply drawing an extra dividing line. The absence of such lines suggests that the break facets in the original may have been unsplit. That said, the author of the replica acknowledges that the drawing itself is likely imperfect in its proportions, and so one cannot conclusively dismiss errors in facet depiction. The debate remains unresolved: the split version tends to yield marginally higher brilliance and is marginally easier to cut, while the unsplit version more closely adheres to what the original drawings show.
This depth of nuance—how a single facet line might alter optical performance—illustrates the challenges inherent to reconstructing diamonds from history’s most famous jewels.
Reconstruction, Lost History, and Rediscovery
Beyond dimension and facet debates, the French Blue’s narrative is dramatic. After centuries of royal prominence, it was stolen in 1792, during the upheaval of the French Revolution, and vanished from official records. Its disappearance is one of the central mysteries in the lineage of diamonds with historic significance.
In December 2007, researchers made a breakthrough: they discovered a lead mould tucked away in the mineral collections of the Muséum national d’Histoire naturelle in Paris. Labeling associated with the mould referenced a “Mr. Hope of London,” which lends weight to theories that the French Blue may have been acquired (in whole or in part) by Henry Philip Hope before it was transformed into what later became the Hope Diamond. The discovery of this mould allowed for more accurate 3D modelling of the French Blue, reconciling some—but not all—of the earlier discrepancies.
Over the period from early 2008 to spring 2009, joint work by Scott Sucher and Dr. François Farges refined the visual representation, dimensions, and optical modelling of what the French Blue likely looked like before its theft. The research suggests that the cutting was significantly more complex than earlier reconstructions allowed—making it a genuine masterpiece of 17th-century gem artistry.
The story also intersects with the Hope Diamond’s emergence: The Hope was first publicly sketched in London in 1812, and its later appearance in the collection of Henry Philip Hope in 1839 hints strongly at a link to the lost French Blue. The rediscovery of the mould and subsequent modelling work strengthen the position of those who contend the two gems share a common origin.
These computer generated images show the appearance of the French Blue diamond. A spectral file of the Hope diamond was imported to generate the exact color of the original diamond. Photoreal image generated using Diamcalc by Octonus (www.octonus.com)
Why This Matters in the Legacy of Famous Diamonds
For enthusiasts of historic diamonds worth millions, the French Blue holds a unique place: it is both a lost gem and a bridge in the genealogy of great blue diamonds. Its narrative ties it to iconic stones like the Hope and Tavernier Blue—making it a central figure in the saga of diamonds with fascinating pasts.
Reconstructing the French Blue is not merely an exercise in geometry—it is an act of resurrecting a piece of gemstone lore. The careful balancing of drawing vs measurement, the interpretation of facets, and the logic of optical performance all play a role in bringing a legendary diamond back into view—if only virtually.
Golden Fleece front
Golden Fleece back
Photo by Scott Sucher