The Hope Diamond appears almost black in its darkest areas. Only when light strikes the facets does its deep greyish-blue colour emerge between the white diamonds of the necklace. The stone weighs 45.52 carats, measures approximately 2.5 × 2.1 × 1.2 centimetres and has a modified antique cushion cut. It is large enough to dominate a jewel, yet its history is larger still. [1][2]
A blue colour with a hidden reaction
In 1988, GIA graded the colour as fancy dark grayish-blue and the clarity as VS1. When the stone was removed from its setting and examined again in 1996, its colour was described as natural fancy deep grayish-blue; the Smithsonian stresses that the difference reflected revised terminology, not a change in the diamond itself. [2][3]
The blue colour is connected to traces of boron in the diamond's crystal lattice. The Hope is a natural type IIb diamond, and under ultraviolet light it reveals a property hidden in daylight: when the UV source is switched off, the stone continues to glow an intense red. This is phosphorescence, an afterglow, rather than the light seen while the UV source is active. GIA has measured the boron concentration at approximately 0.36 ± 0.06 ppm and concludes that the strong blue colour owes more to the stone's unusual size than to an extreme concentration. [4][5]
From French royal blue to the name Hope
The story can be traced to the French merchant Jean-Baptiste Tavernier, who sold a large blue diamond to Louis XIV in 1668. It was recut as the French Blue and entered the Crown Jewels. In September 1792, during the French Revolution, the Crown Jewels were stolen. The blue diamond disappeared. [6]
A smaller, recut blue diamond appeared in London in 1812. In an 1839 catalogue it was recorded in the collection of banker Henry Philip Hope, from whom it took its name. The Smithsonian gives the accepted weights as about 115 modern carats for Tavernier's diamond, about 69 carats for the French Blue and 45.52 carats for the Hope Diamond. The relationship between the stones has since been studied through historical dimensions and models of the earlier cuts.