Type IIb diamond

Type IIb is the rarest natural diamond classification, accounting for fewer than 0.1% of natural gem-quality diamonds. The defining feature is boron substitution in the carbon lattice — boron atoms replace carbon at trace concentrations (typically parts per million) and produce two characteristic properties: electrical conductivity (Type IIb is the only naturally semiconducting diamond) and blue body color.

The blue color is driven by boron's absorption pattern. Boron substitutional defects absorb in the red and infrared, leaving blue and green wavelengths to pass through. The result is the distinctive blue body color that defines the natural blue diamond category. The Hope diamond, the Wittelsbach-Graff, the Oppenheimer Blue, and the De Beers Cullinan Blue are all famous Type IIb naturals.

Type IIb is the only natural diamond type that electrically conducts. The boron substitutions create available electron states that allow current flow at room temperature — a property no other diamond type shares. The conductivity test (a simple resistance measurement) is one of the routes by which a lab can flag a blue stone as natural Type IIb vs treated blue (irradiation-treated, which does not conduct).

Natural Type IIb is overwhelmingly associated with one source: the Cullinan mine in South Africa (formerly Premier mine), which has produced an outsized share of the world's famous blue diamonds. The Argyle mine in Australia (now closed) produced a smaller share. Other deposits have produced Type IIb naturals at much lower rates.

Type IIb naturals carry the largest premium of any diamond type. Top-color blue Type IIb naturals at investment grade trade in the high seven figures per carat at auction; the Oppenheimer Blue sold at Christie's in 2016 for $57.5 million ($3.93 million per carat). Even sub-investment-grade natural blue Type IIb stones trade at multiples of comparable fancy yellow or fancy pink at the same saturation and tone.

The trade distinguishes natural blue Type IIb from treated blue diamond (typically irradiated natural Type Ia, sometimes followed by annealing). Treated blue is disclosed on every GIA Diamond Grading Report as "irradiated" and trades at a small fraction of natural blue Type IIb at the same finished color. Lab-grown blue diamond is also produced — typically HPHT-grown with boron doping, which produces a true Type IIb lab-grown stone with the same electrical conductivity as natural Type IIb. GIA and IGI distinguish lab-grown blue from natural blue via DiamondView fluorescence patterns and growth-feature analysis.

For a buyer, "Type IIb" on a natural blue diamond report is the signature credential — the natural-vs-treated origin call is what makes the blue diamond worth its multi-million-dollar trade.