The Hope Diamond is a 45.52 carats (9.104 g; 0.3211 oz) diamond extracted in the 17th century from the Kollur Mine in Guntur, India. It is blue in color due to trace amounts of boron. Its exceptional size has revealed new information about the formation of diamonds.

The stone is one of the Golconda diamonds. The earliest records of the diamond show that French gem merchant Jean-Baptiste Tavernier purchased it in 1666 as the Tavernier Blue. The stone was cut and renamed the French Blue (Le bleu de France); Tavernier sold the stone to King Louis XIV of France in 1668. It was stolen in 1792 and re-cut with the largest section of the diamond appearing under the Hope name in an 1839 gem catalogue from the Hope banking family.

The diamond has had several owners, including Washington socialite Evalyn Walsh McLean, who was often seen wearing it. New York gem merchant Harry Winston purchased the diamond in 1949, touring it for several years before donating it in 1958 to the Smithsonian National Museum of Natural History in the United States, where it is on permanent exhibition.

Classification

The Hope Diamond, also known as Le Bijou du Roi ("the King's Jewel"), Le bleu de France ("the French Blue"), and the Tavernier Blue, is a large, 45.52-carat (9.104 g; 0.3211 oz), deep-blue diamond, studded in a pendant Toison d 'or.

It is currently housed in the National Gem and Mineral collection at the National Museum of Natural History in Washington, D.C.

It is a dark greyish-blue color under ordinary light because of trace amounts of boron within its crystal structure, and exhibits a red phosphorescence under exposure to ultraviolet light. It is classified as a type IIb diamond.

The Hope Diamond has changed hands numerous times on its way from Hyderabad, India, to France, Great Britain, and the United States, where it is on public display. It has been described as the "most famous diamond in the world".

Physical properties

• Weight: In December 1988, the Gemological Institute of America's laboratory determined the diamond to weigh 45.52 carats (9.104 g; 0.3211 oz).
• Size and shape: The diamond has been compared in size and shape to a pigeon egg or a walnut that is pear-shaped. The length, width, and depth are 25.60 mm × 21.78 mm × 12.00 mm (1 in × 7/8 in × 15/32 in).
• Color: It has been described as being a "fancy dark greyish-blue" as well as "dark blue in color," or having a "steely-blue" color. Blue diamonds similar to the Hope can be shown by colorimetric measurements to be grayer (lower in saturation) than blue sapphires. In 1996, the Gemological Institute of America examined the diamond and, using their proprietary scale, graded it fancy deep grayish blue.The Hope Diamond in the National Museum of Natural History, Washington D.C., 2014. Visually, the gray modifier (mask) is so dark (indigo) that it produces an "inky" effect, appearing almost blackish-blue in incandescent light. Current photographs of the Hope Diamond use high-intensity light sources that tend to maximize the brilliance of gemstones. In popular literature, many superlatives have been used to describe the Hope Diamond as a "superfine deep blue," often comparing it to the color of a fine sapphire—for example, "blue of the most beautiful blue sapphire" (Deulafait)—and describing its color as "a sapphire blue." Tavernier described it as a "beautiful violet".
• Phosphorescence: The stone exhibits an unusually intense, brilliant red phosphorescence after exposure to short-wave ultraviolet light. This 'glow-in-the-dark' effect persists for some time after the light source has been switched off, and this strange quality may have helped fuel its reputation of being "cursed." The red glow is a phenomenon of blue diamonds that helps scientists "fingerprint" them, allowing them to distinguish real ones from artificial ones. The red glow occurs because of a mix of boron and nitrogen in the stone.
• Clarity: The clarity was determined to be VS1, with whitish graining present.
• Cut: The cut was described as being "cushion antique brilliant with a faceted girdle and extra facets on the pavilion."
• Chemical composition: In 2010, the diamond was removed from its setting to measure its chemical composition. After boring a hole one nanometer deep, preliminary experiments detected the presence of boron, hydrogen, and possibly nitrogen; the boron concentration varies from zero to eight parts per million. The boron is responsible for causing the blue color of the stone.
• Touch and feel: When Associated Press reporter Ron Edmonds was allowed by Smithsonian officials to hold the gem in his hands in 2003, he wrote that the first thought that had come into his mind was, "Wow!" It was described as "cool to the touch." He wrote:

You cradle the 45.5-carat stone—about the size of a walnut and heavier than its translucence makes it appear—turning it from side to side as the light flashes from its facets, knowing it's the hardest natural material yet fearful of dropping it.
— Associated Press reporter Ron Edmonds in 2003

• Hardness: Diamonds in general, including the Hope Diamond, are the hardest natural minerals known on Earth, but because of weak planes in the bonds of a diamond's crystalline structure, the crystal can fracture along these planes if not handled correctly. These weak planes, allow diamond cutters to split a rough uncut stone into smaller flawless parts before the process of faceting the stone takes place. Only a diamond can scratch another diamond, so to create a faceted diamond, the uncut rough is mounted in a holder, and then the flat surfaces or facets are ground into the surface of the stone using specially made metal wheels impregnated with diamond particles. These facets are ground and polished using ever finer grades/grits of diamond powder until they have a clear mirror surface, ultimately producing a gem that sparkles by refracting and reflecting light in different ways.

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