Chemistry of inorganic compounds – Changing color characteristic of impurity
Reexamination Certificate
2003-01-08
2004-02-17
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Changing color characteristic of impurity
C423S446000
Reexamination Certificate
active
06692714
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to the production of gem-quality diamonds and more particularly to the production of these gem-quality diamonds from inferior-grade discolored or so-called “brown” diamonds.
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. This category is divided into Type Ia diamonds where the nitrogen exists in an agglomerated state as either pairs called A Centers (Type IaA) or clusters of four nitrogen atoms called B centers (Type IaB) or mixtures thereof (Type IaA/B), and Type Ib where the nitrogen occurs as only isolated single nitrogen atoms called C Centers. Some diamonds also contain clusters of three nitrogen atoms called N3 Centers. Over 98% of the larger clear natural diamonds are Type Ia. Type Ib diamonds are rarer and amount to only 0.8% of natural stones. Type Ia diamonds also contain platelets, which are small flat inclusions a few atoms thick and about 300 atoms across, that 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 which contain nitrogen in an unknown form. Voidites tend to be seen principally in Type IaA/B or Type IaB diamonds.
Generally, it is believed that all nitrogen-containing diamonds started out as Type Ib with isolated nitrogen atoms (C Centers) that were incorporated during crystal growth. During a long period of time (perhaps up to 1 billion years), the diamonds were annealed within earth's mantel at temperatures between 1000° and 1300° C. and at high pressure. During this time, the nitrogen atoms migrated in the diamonds and principally formed two types of aggregates, namely pairs (A Centers) or clusters of four (B Centers).
4C→2A→1B (1)
It is believed that the clusters of four nitrogen atoms (B Centers) are formed when migrating nitrogen pairs (A Centers) collide with each other. Thus, the progression is believed to be Type Ib to Type IaA to Type IaA/B to Type laB. A small amount of nitrogen may also agglomerate as N3 centers which are a planar array of three nitrogen atoms surrounding a common vacancy. It is believed that such centers are formed when an isolated nitrogen (C Center) combines with a nitrogen pair (A Center) during the agglomeration process. N3 centers apparently are less stable than A and B centers as their concentration in Type Ia diamonds is relatively small. Platelets form as soon as the annealing has progressed to the Type IaA stage. Voidite formation, as well as some platelet disintegration, occurs as B clusters form and becomes pronounced in the Type IaB stage of annealing.
Type II diamonds make up approximately 0.7% of all diamonds and contain no nitrogen. Type II diamonds are further divided into Type IIa and Type IIb. Type IIa diamonds have no impurities. Type IIb diamonds contain boron in the parts per million range and are extremely rare, comprising less than 0.001% of all diamonds.
The color of natural diamonds can range from clear and colorless to yellow, orange, red, blue, brown, and even green. For natural diamonds, a brownish tinge is the most common color and may occur in as many as 98% of mined natural diamonds. Type Ia diamonds containing nitrogen can be colorless if all of the nitrogen is tied up in A or B centers. However, if isolated nitrogen atoms (C Centers) or N3 centers are present, the diamonds will have a yellow tinge whose hue depends on the concentration of these forms of nitrogen atoms. Typically, the N3 centers produce the washed-out yellow that is referred to as “Cape Yellow,” while isolated nitrogen atoms (C Centers) produce the richer more vibrant “Canary Yellow” if their concentration is high enough. A small amount of yellow in an otherwise colorless diamond can significantly decrease its market price. However, a rich deep yellow color can produce a “fancy” yellow that has a very high value in the marketplace.
Changing the concentration of N3 centers not only will change the yellow color of a diamond, but can increase the actual brilliance or amount of light thrown back by the diamond. The electrons around an N3 center absorb light in the invisible ultraviolet part of the natural light spectrum, as well as blue light in the visible spectrum. In normal daylight, about {fraction (1/10)} of the energy of the light is in the form of invisible ultraviolet radiation. If the N3 concentration is relatively high, i.e., 100 ppm, then visible blue light is strongly absorbed and the diamond will have a definite yellow color, which will lower its value. However, if the concentration of N3 centers is reduced by some treatment so that the yellow coloring disappears, the remaining N3 centers can affect the brilliance of a diamond by a two-stage process. First, an invisible ultraviolet photon is absorbed by an N3 center. The energy is temporarily stored in the N3 center. Some of this energy leaks away in the form of phonons or lattice vibrations. After a storage time pre-determined by the half-life of the center, the N3 center will re-emit the remaining energy as visible light. Since some energy has been lost, the re-emitted light is no longer in the high-energy invisible ultraviolet part of the spectrum. Instead, the re-emitted light now is in the visible spectrum (the technical term for this is “ultraviolet downshifting”). Because we do not see ultraviolet light, we do not notice that it is being absorbed (an animal, like a bee that can see ultraviolet light, would see the brilliance of the diamond decreased by the absorption of ultraviolet light by N3 centers). All we see is the increased emission in the visible spectrum and, thus, the diamond now appears extraordinarily bright. Consequently, a controlled reduction of N3 centers in a Type Ia diamond by any treatment will increase the value of a diamond containing them in two ways. First, elimination of some N3 centers reduces or eliminates the yellow tinge in the diamond. Second, the remaining N3 centers will increase the brilliance of the diamond relative to a perfect Type IIa diamond.
Most Type Ia diamonds as mined are of a brownish color. A brown color can result from the mixture of many other fundamental colors. One way is to mix some yellow coloring from isolated nitrogen atoms (C Centers) or N3 centers with some black color, perhaps from submicroscopic inclusions of graphite. The mixture of yellow and black will produce a brown color. Another way to make a brown diamond is to mix a color center that produces a green diamond with a color center that produces a red diamond. The combination of red and green again will produce a brown color. In fact, there are an infinite number of color combinations that will produce a brown color. Thus, it is not possible to determine the color centers causing the color of a diamond by its color. However, the reverse process is unique; that is, if one knows the type and concentration of color centers in a diamond, one can predict the resulting color.
Type II diamonds vary from colorless to a deep blue color. Type II diamonds are most valuable when they are colorless or a pure-blue color. Excessive mechanical deformation and plastic flow are believed to cause many Type II diamonds to have a reddish brown or pink tinge which lowers their value considerably. Many natural Type IIa diamonds have this color tinge and their value could be greatly enhanced as jewelry if they could be made colorless. Some Type IIa diamonds have a steel gray haze in them that also greatly decreases their value. Generally, in the diamond trade, a discolored diamond whether it is brownish, reddish brown or steel gray is labeled as a “brown” diamond. Previous attempts to treat Type IIa diamonds to increase their value have failed. G. Lenzen,
Diamonds and Diamond Grading
, p. 207
Anthony Thomas Richard
Banholzer William Frank
Jackson William Edwin
Kaplan George Rene
Vagarali Suresh Shankarappa
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