In the few recorded cases in which a change of color has to the organic nature of the coloring matter.
The coloring of many diamonds is so faint that an unpracticed observer, unless he is able to compare such a stone with an absolutely colorless diamond or to place it against a background of pure white, will fail to recognize that the stone is colored at all. The practiced eye of the diamond merchant, however, needs no such assistance in recognizing the most faintly colored stones. Such stones are rather lower in value than absolutely colorless specimens of the same clearness and transparency, but the difference in price is not very considerable. The shades of color, which appear most frequently, are light yellow, gray, and green. A faintly yellow diamond is not observable as such in any artificial illumination other than the electric light, and then appears to be a colorless stone. Diamonds of a faint bluish tinge are known, but are much less common.
As mentioned above, diamonds showing a pronounced coloration constitute about one-half of the total output. Almost all the colors of the mineral kingdom may be represented in numerous and varied tints, so that the suite of colors of the diamond is very extensive.
A magnificent collection of differently colored diamonds, the most beautiful and the richest in existence, was preserved in the treasury of the royal palace at Vienna. Helmreichen, who spent many years in Brazil, and was so enabled to make the series very complete, brought it together.
The color, which occurs most frequently in diamonds, is yellow, in various shades, such as citron-yellow, wine-yellow, brass-yellow, ochre-yellow, and honey-yellow, but golden-yellow has not as yet been observed. Most of the Cape diamonds are colored with one or other of these tints of yellow. After yellow, green is the most commonly occurring color, especially in Brazilian diamonds.
Green or yellowish-green is seen most frequently, then pale green, leek-green, asparagus-green, pistachio-green, olive-green, emerald-green, bluish-green and grayish-green. Brown diamonds are also common at all localities; the different shades are light brown, coffee-brown, clove-brown, and reddish-brown. Shades of gray, such as pale gray, ash-gray, smoke-gray, are not rare.
The different shades of red, a color, which is rarely met with in diamonds, are lilac-red, rose-red, peachblossom-red, cherry-red, and hyacinth-red.
Blue in its two shades, dark blue and pale sapphire-blue, is the rarest of all colors to be met with in diamonds.
The coloring of diamonds is seldom intense, pale colors being much more usual than deeper shades. Diamonds which combine great depth and beauty of color, with perfect transparency, are objects of unsurpassed beauty; for, in addition to their fine color, they possess the wonderful luster and brilliant play of prismatic colors peculiar to the diamond, so that other finely-colored stones, such as ruby and sapphire, are not to be compared with them. Only a few stones of this description are in existence; they are among the most highly prized of costly gems.
Of such deeply colored and perfectly transparent diamonds, bright or deep yellow specimens are, since the discovery of the South African diamond fields, the least rarely met with. The largest of these yellow Cape diamonds is a beautiful orange-yellow brilliant, weighing 125 3/8 carats, and was in the possession of Tiffany and Co., the New York firm of jewelers. A few fine yellow stones dating back to ancient times were preserved in the "Green Vaults" at Dresden.
Diamonds of a fine green color are distinctly rare, only a few Examples being known; the same may be said of red diamonds and, even more emphatically, of blue diamonds. The most beautiful green diamond known is a transparent brilliant weighing 48 carats, which was preserved in the "Green Vaults" at Dresden; it will be again mentioned in the section devoted to famous diamonds. Tschudi mentions two beautiful specimens from Brazil, one of an emerald-green and the other of a sea-green color, while the existence of other Brazilian stones with a color very similar to that of the yellowish-green of uranium glass, but inclining more to yellow, is mentioned by Boutan.
Red stones are rare. The ten-carat ruby-red stone, which belonged to Czar Paul I. of Russia, and which is said to be still preserved with the Russian crown jewels, is often mentioned as an example of a red diamond; nothing more definite concerning it is, however, known. A better-authenticated example is the "Red Haiphen" diamond, a ruby-red brilliant weighing one carat, while recently Streeter has reported the discovery of a beautiful red stone in Borneo, and its sale in Paris. Several Examples of beautifully transparent rose-red diamonds are known; such, for instance, as that of the fifteen-carat stone belonging to the Prince of Riccia, a few smaller specimens in the treasury at Dresden, and a thirty-two-carat stone, the most beautiful rose-red known, in the treasury of Vienna. A rose-colored brilliant, called the "Fleur de Pecher," is (was) among the French crown jewels, and Tschudi mentions a peachblossom-red stone from the Rio do Bagagem, in Minas Geraes, Brazil.
Blue diamonds are the most rare of all. A magnificent blue brilliant of 44 1/4 carats, the "pearl of colored diamonds," was formerly in the possession of Mr. Hope, a London banker. It is probably a portion of Tavernier's blue diamond of 67 1/8 carats, stolen in 1792 with the French crown jewels. A small diamond of a deep blue color, and a pale blue one of forty carats, were preserved in the Munich treasury.
Black diamonds perhaps deserve a brief mention. Crystals of a uniform black color have been found in Borneo, and also very rarely in South Africa. The opacity of such stones, combined with their high degree of luster, almost metallic in its character, render them, when cut, of peculiar beauty, and well suited for use in mourning jewelry. These crystals of black diamond must not be confused with the black carbonado of Brazil, to be described later. A few brown stones of a delicate and beautiful coffee shade are known; these also come from Brazil.
As is most always the case with precious stones and other minerals, the color of which is due to enclosed foreign matter in an extremely fine state of division, so also in the diamond the distribution of the coloring matter is not always perfectly uniform throughout its whole mass.
Not infrequently the bulk of a rough diamond is colorless, the edges and corners only being colored; many Brazilian stones are of this description, as well as specimens from South Africa, including some of the "smoky stones" already mentioned. In these the deep smoky-gray color is sometimes confined to the corners of the stone, the interior being faintly colored or entirely colorless; a stone so colored is described as a "glassy stone with smoky corners." Diamonds in which these conditions are reversed also occur, the edges and corners being colorless, and the central portion colored.
Sometimes, though rarely, a stone shows two differently colored portions; the two portions of one mentioned by Mawe were colored respectively yellow and blue. Stones showing a number of differently colored sectors, sharply separated from each other, and radiating from a central point, are also of rare occurrence. Thus smoke-grey and colorless rays may have a regular star like arrangement, or may form a figure like the club of playing cards, on the faces of the octahedron.
Of interest is the fact that diamond sometimes shows a play of colours like that of precious opal. Des Cloizeaux mentioned a few such stones, which differed from opal in this respect only in that the colours were less brilliant. Pale blue and yellow stones have been reported by Mawe to show a somewhat similar appearance.
The color of diamonds is by no means in every case unchanging and unalterable. Some stones are bleached by sunlight; thus a red diamond on exposure to sunlight is reported to have gradually lost its color and become white. A diamond in the possession of the Parisian jeweler, Halphen, undergoes a peculiar change in color on exposure to heat. This stone, which is of a faint brownish color and weighs four grams (about twenty carats), assumes in the fire a beautiful rose-red color. If kept in darkness this color is retained for about ten days, after which it returns to its original brownish color; should the stone be exposed to diffuse daylight, or to the direct rays of the sun, the change to the original color is much more rapid. The change to rose-red can be produced at will by again exposing the stone to the action of heat. Could means be devised for retaining the rose-red color of the stone, the possessor would benefit to the extent of many thousand francs, since it is valued when brown at 60,000 francs, and when rose-red at from 150,000 to 200,000 francs. Halphen has also seen a diamond which when rubbed assumed a rose-red color:
The color of diamonds is in some cases affected by exposure to a high temperature. According to Des Cloizeaux, pale green diamonds, after being heated in the ox-hydrogen flame, became light yellow; brown crystals under the same conditions become grayish. Baumhauer also witnessed the color of one diamond change from green to yellow, and of another from dark green to violet, under the influence of a high temperature. Wöhler caused green diamonds to assume a brown color by heating them; but found that brown stones remained unaltered in color. Yellow diamonds, especially those from the Cape, retain their color at the highest temperatures.
It has already been mentioned that very faintly colored stones command a somewhat lower price than do those, which are perfectly colorless. Hence many attempts have been made to transform faintly colored stones into the more valuable colorless stones. This is readily effected in those Brazilian diamonds, which have a colorless central portion surrounded by a colored external layer of no great thickness. The outer colored layer is in these cases simply burnt away by heating the stone in a crucible with a little saltpeter. The operation is very brief, the colored external layer disappearing in one or two seconds. This device involves no actual change of color, but is simply a removal of the colored portions, which could have been effected just as well by the more lengthy process of grinding.
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting Diamond trade
Rafal Swiecki, geological engineer email contact
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