Optics: measuring and testing – Crystal or gem examination
Reexamination Certificate
1999-10-29
2002-04-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Crystal or gem examination
C356S244000, C250S372000, C250S330000
Reexamination Certificate
active
06377340
ABSTRACT:
BACKGROUND OF THE INVENTION
The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.
The invention relates to a method for detecting processed gem-quality colorless and fancy-colored diamonds. In particular, the invention relates to a method for detecting processed diamonds that have been produced from inferior-grade discolored diamonds by a high-temperature and high-pressure (HPHT) process.
Diamonds are conventionally divided into four main categories, which are designated as Type Ia, Type Ib, Type IIa, and Type IIb. In reality, there is a smooth change in impurity concentration/arrangement between the four types so that intermediate varieties thereof also exist.
Type I diamonds contain nitrogen as the major impurity, and can be divided into Type Ia diamonds and Type Ib diamonds. Type Ia diamonds contain a nitrogen impurity that exists in an agglomerated state. The agglomerated state exists as nitrogen pairs, called “A Centers” (Type IaA), nitrogen clusters comprising four nitrogen atoms called “B centers” (Type IaB), and mixtures thereof (Type IaA/B). Type Ib diamonds contain nitrogen as isolated single nitrogen atoms called “C Centers”. Some Type I diamonds may also contain clusters of three nitrogen atoms called “N3 Centers”.
Type Ia diamonds comprise over about 98% of the larger clear natural diamonds. Type Ib diamonds are rarer and amount to only about 0.8% of natural diamonds. Type Ia diamonds may also contain platelets, which are small flat inclusions that a few atoms thick and about 300 atoms across. The platelets may contain some nitrogen in an unspecified form. Type Ia diamonds also may contain voidites, which are small equiaxed cavities that are either vacant or that contain nitrogen in an unspecified form. Voidites tend to typically exist in Type IaA/B diamonds or Type IaB diamonds.
Natural diamonds may possess a color that can range from clear and colorless diamonds to yellow, orange, red, pink, blue, brown, and even green colored diamonds. Intermediate colored diamonds are also possible. Diamonds can even appear to change color depending on the lighting conditions. These diamonds are known in the art as “chameleon” diamonds. For natural diamonds, a brownish color is the most common, and may occur in up to about 98% of mined natural diamonds. The brownish and pinkish color is believed to be a result of plastic deformation of the diamonds after they were formed.
Most natural Type Ia diamonds have a brownish color. A brownish color may result from a mixture of many other colors. For example, the brownish color may result from a mixture of yellowish coloring (such as from isolated nitrogen atoms (C Centers) or N3 centers) with some blackish coloring (such as from submicroscopic inclusions of graphite). The mixture of yellowish and blackish colorings will produce a brownish color. Further, a brownish coloring in a diamond can be formed from a mixture of color centers that produce a greenish coloring in a diamond with a color center that produces a reddish coloring in a diamond. An infinite number of color combinations that produce a brownish color is possible. Therefore, it is generally impossible to determine the color centers causing the color of a diamond by its color alone.
Type II diamonds are conventionally defined as diamonds that contain less than 1 PPM of nitrogen. The selection of 1 PPM nitrogen level is historically related to the fact that this was the level of nitrogen routinely detectable in infrared spectrums of gem diamonds. Type II diamonds are further divided into Type IIa and Type IIb. Type IIa diamonds have no other impurities other than nitrogen at less than a 1 PPM level. Type II diamonds contain boron in the parts per million range and the boron concentration always exceed any nitrogen concentration in the crystal. Type IIb diamonds are blue in color and are extremely rare, and thus have a high value per carat as jewelry items.
The pricing of diamonds typically is a function of their color. Diamonds that exhibit fancy colors, such as the canary yellows, blues, reds, pinks, and greens, are rare and tend to have the highest prices. Diamonds that are classified as “colorless diamonds” command the highest prices after fancy colored diamonds. The degree of colorlessness lends to a nonlinear price effect of the diamond. Even the faintest tinge of yellow can considerably reduce the price of colorless diamonds. Brownish diamonds are an exception to the above-stated fancy color diamond market, as they are very common. Brownish diamonds typically have been culled and used as industrial diamonds, and thus the brownish diamonds are relatively inexpensive.
An aesthetic and economic incentive thus exists to change relatively inexpensive brownish colored diamonds to either of the more valuable colorless diamonds or to fancy color diamonds in view of the relative scarcity and beauty of fancy colors, colorless, and the commonality of brownish diamonds. Various methods have been proposed and used to treat diamonds. For example, irradiation has been used to change the diamond color from typically unattractive off-colors to attractive blue, green, orange, black, and yellow colors. Also, electrons, neutrons, gamma rays, and alpha particles have been proposed and used to produce irradiation-produced color centers in diamonds. Neutron, gamma, and electron irradiation have been generally used because these methods produce a more uniform coloration in diamonds due to an effective penetrating power into the diamonds. However, neutrons used for treating diamonds may introduce dangerous side effects since radioactive species can be produced in diamond inclusions by neutron activation. Additionally, typical electron or alpha irradiation methods merely develop a superficial color that is confined to outer portions of the diamonds.
Further, methods such as, but not limited to, laser-drilling, fracture-filling, and surface-coating have been proposed and used to treat diamonds in attempts to increase their value. These methods, while somewhat effective in the color change, are easily detected by conventional gemological practices. For example, an observation of diamonds in an optical microscope may reveal treating by laser-drilling, fracture-filling, and surface-coating methods.
Most diamond treating is easily detected by looking at infrared and optical spectrums of the diamonds. The treating produces infrared and optical spectrum characteristics (also known as “detection signatures”), which include detectable different radiations that are produced by vacancies in diamonds. The detection signatures or optical spectra signature characteristics are observed in the GR1 band of the visible spectrum; and absorption at 740.9 nm and from 412-430 nm by the GR1 band that produces a green, blue-green, dark green, or even a black color in the diamond by absorption.
Vacancy color centers introduced by irradiation may be modified by high-temperature annealing treatments to produce diamond colors that range from blue to pinkish to red to green colors. Annealing can be conducted at temperatures as low as about 600° C., because the large number of vacancies introduced by irradiation temporarily increases the mobility of nitrogen and other impurities in the diamonds. During the annealing, the vacancies diffuse to, and are absorbed by at least one of vacancy sinks in the diamond, such as, but not limited to, free surfaces, dislocations, and inclusion interfaces, and complexes of nitrogen, such as A, B, and C Centers in the diamonds. As the vacancies disappear, their immediate influence on the observed diamond color lessens. Thus, the diamond color gradually changes from blue to green to brownish to yellow and back to the original color of the diamond. The annealing can be stopped at any point during the annealing to produce a desired treated diamond color. Multiple irradiation steps and annealing steps may be conducted to manipulate and ch
Anthony Thomas Richard
Casey John Kieran
Smith Alan Cameron
Vagarali Suresh Shankarappa
Font Frank G.
Johnson Noreen C.
Nguyen Sang H.
Santandrea Robert
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