Natural Fancy Blue Diamond
Natural Fancy Blue Diamond
Boron-coloured Type IIb diamonds — among the rarest and most expensive of all coloured stones
A natural fancy blue diamond is a diamond whose blue colour arises from trace boron substituting for carbon in the crystal lattice. The classification is Type IIb, defined by the presence of boron and the relative absence of nitrogen. Boron acts as an electron acceptor in the diamond lattice, and the resulting absorption removes red and yellow wavelengths from transmitted light, producing the characteristic blue body colour. Type IIb diamonds are also semiconductors, a property that distinguishes them from all other diamonds and provides one of the diagnostic tests used in laboratory identification.
Cause of colour and Type IIb
Boron concentrations in natural blue diamonds are extraordinarily low, typically in the range of 0.1 to 1 part per million by weight. Such low concentrations are sufficient to produce visible colour because each boron atom creates an electronic acceptor level a few tenths of an electron-volt above the valence band, and the absorption associated with this level extends across the red and infrared portions of the spectrum. The colour seen face-up depends on the relative concentrations of boron, residual nitrogen, and any other impurity defects, and ranges from grey-blue (most common) through pure blue (rare) to violet-blue (rarest).
The Type IIb classification is determined by infrared spectroscopy: the spectrum shows the boron-related absorption at 2802 wavenumbers (cm⁻¹) and lacks the nitrogen-related absorptions of Type Ia and Type Ib. The semiconducting property — Type IIb diamonds conduct electricity at room temperature, while all other diamonds are insulators — provides a quick non-destructive test that distinguishes natural blue Type IIb from blue diamonds whose colour comes from other causes (irradiated diamonds, hydrogen-rich grey-blue diamonds, etc.).
Sources
Most known natural fancy blue diamonds come from a small number of mines. The Cullinan mine in South Africa (formerly the Premier mine) is the dominant historical and contemporary source, having produced the Cullinan I and II diamonds (white) and a steady supply of fancy blue stones across more than a century of production. The Argyle mine in Western Australia, closed in 2020, produced a small number of blue diamonds among its predominantly pink and brown output. The Golconda region of India produced historic blue diamonds including the Hope Diamond (45.52 carats), the Wittelsbach-Graff (31.06 carats), and others; Indian production effectively ceased in the eighteenth century.
Smaller numbers of natural fancy blue diamonds have come from the Jwaneng mine in Botswana, the Letseng mine in Lesotho, and various Russian sources. The geological setting that produces Type IIb diamonds is unusual: current research suggests the boron is incorporated at lower-mantle depths far below the depth at which most gem diamonds form, possibly involving subducted oceanic-crust boron sources.
Famous examples
The Hope Diamond, at 45.52 carats, is the most famous natural fancy blue diamond. It is held by the Smithsonian National Museum of Natural History in Washington, D.C., and shows the characteristic deep grey-blue body colour and a distinctive red phosphorescence under shortwave ultraviolet exposure. The Wittelsbach-Graff (31.06 carats), the Oppenheimer Blue (14.62 carats, sold for $57.5 million in 2016), the Blue Moon of Josephine (12.03 carats, sold for $48.4 million in 2015), and the De Beers Cullinan Blue (15.10 carats, sold for $57.5 million in 2022) are among the most-photographed and best-documented examples.
Grading and rarity
GIA grades natural fancy blue diamonds on the fancy-colour scale: Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Vivid, and Fancy Deep / Fancy Dark. Most natural blue diamonds receive grades in the lower portion of the scale; the Fancy Vivid and Fancy Deep stones are extremely rare. Modifiers — greenish-blue, greyish-blue, violet-blue — are noted on reports and have substantial effect on price. Pure-blue stones without grey or green modifier are the most desirable and command the highest prices.
Natural fancy blue diamonds represent less than one tenth of one per cent of all natural fancy-colour diamonds and a much smaller fraction of total natural-diamond production. Per-carat prices for Fancy Vivid Blue stones above one carat routinely exceed $1,000,000 per carat, and the major auction-house records for any diamond at any size have repeatedly been set by natural fancy blue or fancy blue-related sales.
Distinction from treated and synthetic blue diamonds
Treated blue diamonds — typically irradiated transparent diamonds with electron or neutron exposure — are common in the trade and are priced at a small fraction of natural blue prices. Laboratory testing distinguishes them from natural Type IIb by the spectroscopic signature, the pattern of colour distribution within the stone (irradiated stones often show colour concentrated near the surface), and the absence of the boron-related absorption and electrical conductivity. CVD and HPHT laboratory-grown blue diamonds are also produced, and laboratory identification distinguishes them from natural Type IIb by growth-feature spectroscopy and by careful analysis of fluorescence and phosphorescence patterns.