HIGH PRESSURE/HIGH TEMPERATURE PRODUCTION OF COLORLESS AND FANCY COLORED DIAMONDS

Abstract

The present invention is directed to a method for treating discolored natural diamond, especially Type IIa diamond and Type IaA/B diamond with nitrogen as predominantly B centers, for improving its color. The method includes placing a discolored natural diamond in pressure transmitting medium powder which is consolidated into a pill. Next, the pill is placed into a high pressure/high temperature (HP/HT) press at elevated pressure and elevated temperature within the diamond stable range of the carbon phase diagram for a time sufficient to improve the color of said diamond. Finally, the diamond is recovered from said press. Colorless diamond can be made by this method.

Description

Brief Summary of Invention

[0018] The present invention is directed to a method for treating discolored natural diamond, especially Type IIa diamond and Type Ia diamond with nitrogen as predominantly B centers, for improving its color. The method includes placing a discolored natural diamond in pressure transmitting medium powder which is consolidated into a pill. Next, the pill is placed into a high pressure/high temperature (HP/HT) press at elevated pressure and elevated temperature within the diamond stable range of the carbon phase diagram for a time sufficient to improve the color of said diamond. Finally, the diamond is recovered from said press. Colorless Type Ia and IIa diamond can be made by this method.

[0019] Typical temperatures range from about 1500ºto about 2700º C with corresponding pressures ranging from about 5 to about 20 GPa. Times can range from as short as about 30 seconds up to as long as 96 hours or more with times advantageously ranging from around 5 minutes up to 24 hours. These conditions (time, temperature, and pressure) are correlated and adjusted to the nature of the discoloring defects in the diamond which have to be altered in order to improve the color of the diamond.

[0020] Advantages of the present invention include the ability to upgrade the color of discolored or off-colored diamond to produce fancy color diamond or colorless diamond. Another advantage is the ability to maintain the mechanical and structural integrity of the color-improved diamonds. A further advantage is the specific ability to make colorless diamond form Type IIa diamond. A yet further advantage is the ability to retain the optical clarity of the treated diamond disclosed herein. These and other advantages will be readily apparent from the disclosure set forth herein.

Brief Description of Drawings

[0021] For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, wherein:

[0022] Fig. 1 is a cross-sectional view of a conventional HP/HT apparatus which may utilized to anneal discolored diamond for improving their color.; and

[0023] Fig. 2 is a cross-sectional view of a typical reaction cell for annealing natural diamond in the apparatus in Fig. 1

Detailed Description of the Invention

[0024] In the discourse to follow, the precepts of the present invention are illustrated in connection with a conventional HP/HT apparatus which may be of the belt- or die-type described, for example, in U.S. Pats. Nos. 2,947,611; 2,941,241; 2,941,248; 3,609.818; 3,767,371; 4,289,503; 4,673,414; and 4,954,139, the disclosures of which are expressly incorporated herein by reference. However, it will be appreciated that the method of the present invention will find applicability in any HP/HT apparatus of the general type which is capable of providing the required HP and HT conditions simultaneously. Accordingly, it is intended that such other HP/HT apparatuses are within the scope of the invention herein described.

[0025] Looking then to Fig. 1, an illustrative HP/HT apparatus or press useful in conjunction with the present invention is shown generally at 10 to comprise a generally cylindrical reaction cell assembly, 12, interposed between a pair of punches, 14a and 14b, and surrounded by a generally annular belt or die member, 16. Preferably, both punches 14 and belt member 16 are formed of a relatively hard material, such as cemented tungsten carbide. Between punches 14 and belt member 16 are a pair of insulating assemblies, 18a and 18b, each of which is formed of a pair of thermally- and electrically-insulating members, 20a-b and 22a-b, each preferably formed of pyrophyllite or the like, and having an intermediate metallic gasket, 24a and 24b, disposed therebetween.

[0026] As shown, reaction cell assembly 12 includes a hollow cylinder, 26, which may be formed of a material, such as salt or the like, which is converted during HP/HT by phase transformation or compaction to a stronger, stiffer state, or, alternately of a talc material or the like which is not so converted. In either case, the material of cylinder 12 is selected as being substantially free of volume discontinuities or the like, under HP/HT as may occur, for example, with pyrophyllite or alumina materials. Materials meeting such criteria are described in U.S. Pat. No. 3,030,662, the disclosure of which is incorporated herein by reference.

[0027] Positioned concentrically within salt cylinder 26 is an adjacent cylinder, 28, which is provided as a graphite electrical resistance heater tube. Electrical connection with heater tube 28 is achieved via an adjacent pair of conductive-metal end discs, 30a and 30b, which are axially-disposed with respect to heater tube 28. Adjacent each disc 30 is provided an end cap assembly, shown generally at 32a and 32b, each of which comprises an insulating plug, 34a and 34b, surrounded by an electrically conductive ring, 36a and 36b.

[0028] It will be appreciated that the interior of heater 28, along with end discs 30, salt cylinder 26, and end cap assemblies 32, defines a generally-cylindrical inner chamber, shown at 38, having defined axial and radial extents and containing a pressure transmitting medium, 40. Pressure transmitting medium 40 is selected as having a relatively low coefficient of internal friction to make it semi-fluid at HP/HT conditions, and may be provided as a cylindrical salt liner, 42, which defines radial pressure transmitting medium layers 43a and 43b, and is fitted with an axial pair of salt plugs, 44a and 44b, each of which defines an axial pressure transmitting medium layer. Preferably, salt liner 42 and plugs 44 are formed of a graphite material or of sodium chloride, but also may can be formed of any chloride, iodide, or bromide of sodium, potassium, or calcium or a mixture thereof. Alternatively, pressure transmitting medium 40 may be provided in powdered or particulate form. In either case, medium 40 defines a cavity space, as is shown at 46, which is configured to receive the discolored diamond to be annealed. Such is the configuration of a representative HP/HT apparatus for practicing the present invention.

[0029] In departing from conventional uses of HP/HT apparatuses, the diamond to be annealed is placed within a powdered pressure transmitting medium which then is consolidated or densified to in excess of 90% of its theoretical density to form a pill. The pill medium must transmit hydrostatic pressure without loss as a continuum onto diamond surfaces in the HP/HT apparatus to avoid shear stresses that could plastically deform the diamond being treated. The pill medium also must be thermally and chemically stable, and preferably should not react with or cause dissolution of diamond. Suitable media are stable fluids or gases at annealing conditions or highly plastic solids, including, but not limited to, salts such as describe above, oxides such as magnesium or calcium oxide, or carbons such as graphite. The pill medium also must be scaleable to the high pressures and high temperatures that are encountered with the HP/HT apparatus. Finally, the pill medium must possess a volume compressibility which is small and comparable to the gasketing system, i.e., it must be void-free and close to its theoretical lattice density at annealing conditions. Multiple pills can be housed within cavity 46 if necessary, desirable, or convenient.HP/HT conditions for the present invention comprehend conditions such that the diamond is thermodynamically stable. Typically, this includes a temperature in the range of about 1500ºto 2700ºC and a pressure in the range of about 5 to 20 Gpa. Annealing conditions depend upon the nature of the defect in the diamond which have to be removed or changed to improve color. HP/HT conditions are within the diamond stable range of the carbon phase diagram.

[0030] HP/HT conditions are maintained for a time adequate for the color of the discolored diamond to improve. Such times can range from minutes to several hours, more typically about 5 minutes to 24 hours, or even more typically about 10 minutes to 2 hours. In this regard, the discolored diamond can be subject to multiple annealing steps at the same or at different conditions, as the examples will demonstrate.

[0031] Discolored diamonds subjected to the inventive process may be cut and polished prior to annealing or following their recovery from the pill. Thereafter, the diamond is ready for use in jewelry or fabrication into devices, such as, for example, heat sinks, optical windows, or the like. Of special note, is the ability of the present invention to convert discolored Type IIa diamond into a colorless stone.

[0032] The following examples show how the present invention has been practiced, but should not be construed as limiting. In this application, all units are in the metric system, unless otherwise stated. Also, all citations referred to herein are expressly incorporated herein by reference.

[0033] EXAMPLES

[0034] IN THE EXAMPLES

[0035] The annealing of natural diamonds was done under high temperature and high pressure conditions using a belt type apparatus which is capable of reaching very high pressures (˜90 kbars) and temperatures (˜2600ºC). A typical procedure for such diamond annealing as reported in the examples follows.

[0036] The reaction cell assembly is schematically shown in Fig. 2. Diamond crystal 51 is placed inside cylindrical pill 52 which is made of high purity graphite or sodium chloride powder. Graphite is preferred because it does not melt during high temperature annealing. Pill 52 has the following dimensions: diameter of 0.450 (11.43 mm) and height of 0.500 (12.7 mm). Pill 52 is made by compacting the graphite powder and diamond crystal 51 in a hydraulic press. Pill 52 is placed within magnesium oxide tube 53 which is fitted with end discs 54a and 54b. This assembly, then, is placed within graphite tube 55 which is fitted with end discs 56a and 56b. Such assembly is placed within salt cylinder 59 whose ends are fitted with graphite pins 58a and 58b surrounded by, respectively, salt cylinders 60a and 60b. Graphite discs 57a and 57b, respectively, seal the ends of salt cylinder 59.

[0037] The reaction cell assembly then is placed inside a high pressure apparatus (such as a belt-type apparatus) using standard gasket assemblies to form a seal at high pressure and to make electrical contact with the punches so that it can be heated with electrical power.

[0038] A typical annealing run starts with the reaction cell being pressurized to a set pressure of ˜80 kbars. The electrical power is turned on when the pressure reaches ˜96% of the set pressure. Initially, the diamond is heated to ˜1200º C and held at this temperature for one minute. Thereafter, the temperature is increased to the set value, ˜2300ºC, in 2 minutes and held at the set value for 18 minutes. Then, the temperature is slowly reduced to 500º C in 5 minutes and the power turned off. The cell pressure is held constant for 45 seconds after the power is turned off and the pressure then slowly released. The reaction cell is removed from the high pressure apparatus and the graphite pill containing the diamond is removed. The diamond is recovered by digesting the pill in a boiling mixture of 90% sulfuric acid and 10% nitric acid.

[0039] EXAMPLE I

[0040] A Type IIa natural diamond (Stone No. K11) weighing 244 milligrams and being steel brown in color was embedded inside a graphite pill made by pressing high purity graphite powdered. The diamond was annealed at ˜80 kbars and 2300º C for 18 minutes. The diamond crystal was recovered and visual examination showed that its color had changed to clear or colorless. The change in color became more evident after polishing flats on the surface of the annealed diamond.

[0041] EXAMPLE II

[0042] A Type Ia natural diamond (Stone No. F 1816) weighting 210 milligrams and light brown in color was placed inside a pill made by pressing high purity sodium chloride powder. The diamond was annealed at ˜80 kbars and ˜2400ºC for 18 minutes. Visual examination of the recovered diamond showed that the color had changed to golden yellow.

[0043] EXAMPLE III

[0044] A type Ia natural diamond (Stone No. K38) weighing 160 milligrams and medium brown in color was placed inside a pill made by pressing high purity sodium chloride powder. The diamond was annealed at ˜77 kbars and ˜1900º C for 18 minutes. Visual examination of the recovered diamond showed that there was no change in color. The diamond was annealed again at ˜79 kbars and ˜2050º C for 18 minutes which resulted in a change in color to light yellow. The diamond was annealed for a third time at ˜79 kbars and 2200ºC for 18 minutes which resulted in a change in color to greenish-golden yellow. The spectrum of this stone in the UV, visible, near infrared, and infrared spectra showed no unusual absorption lines that are not seen in natural diamond stones. Specifically, the normal irradiation signatures, such as the GR1 line at 741 nm and the H1b and H1c lines arising from a combination of irradiation and heat treatment were absent, as well as was an absorption peak in the near infrared at 985 nm that had been previously seen by GIA laboratory in a greenish-yellow stone that had been treated. Gems and Gemology, supra.

[0045] EXAMPLE IV

[0046] A Type Ia natural diamond (Stone No. K40) weighing 118 milligrams and dark brown in color was placed inside a pill made by pressing a mixture containing 96% high purity sodium chloride and 4% potassium nitrate powders. The diamond was annealed at ˜80 kbars and 2300º C for 18 minutes which resulted in a change in color to greenish golden yellow. The spectrum of this stone in the UV, visible, near infrared, and infrared spectra showed no unusual absorption lines that are not seen in natural diamond stones. Specifically, the normal irradiation signatures, such as the GR1 line at 741 nm and the H1b and H1c lines arising from a combination of irradiation and heat treatment were absent, as well as was an absorption peak in the near infrared at 985 nm that had been previously seen by GIA laboratory in a greenish-yellow stone that had been treated. Gems and Gemology, supra.

[0047] EXAMPLES V

[0048] Three type Ia diamonds, K58-12, K58-61, and K58-62, weighing 31.6, 27.1, and 24 milligrams with initial colors of light brown, dark brown, and dark brown, respectively, were placed inside a pill by pressing a high purity graphite. The diamonds were IaA/B type with nitrogen present in A and B centers. However, in both the K58-61 and K58-62 diamonds, nitrogen was present mainly in B centers. The diamonds were annealed at 79 kbar and 2300º C for 18 minutes. Visual examination of the recovered diamonds showed that K58-12 had changed to a deep yellow, K58-61 to a very light yellow, and K58-62 had changed to colorless.

[0049] EXAMPLES VI

[0050] Three type Ia diamonds, K58-10, K59-40, and K59-60, weighing 26.4, 21.6, and 30.3 milligrams with initial colors of light brown, brown, and brown, respectively, were placed inside a pill by pressing a high purity graphite powder. K58-10 was a type IA/B diamond, whereas both K59-40 and K59-60 were type IaA/B diamonds. The diamonds were annealed at 79 kbar and 2000º C for 18 minutes. Visual examination of the recovered diamonds showed that K58-10 had become colorless, whereas both K59-40 and K59-60 had no change in color.

[0051] Examples V and VI indicate that some type Ia diamonds can be made colorless by annealing under high pressure and high temperature conditions.

[0052] EXAMPLES VII - XX

[0053] Additional diamond samples were annealed by the procedures described above. The results of such annealing procedure, along with the previous examples, is set forth in the following table in which the temperatures and pressures reported are estimated values as is common in the HP/HT industry.

TABLE I
		Color Before		Pressure	Temperature	Time	Color After
Stone No.	Diamond Type	Annealing	Medium	(Kbar)	(° C.)	(min)	Annealing
K11	IIa	steel brown	graphite	80	2300	18	colorless
K18	Ia	light brown	graphite	80	2300	20	pale green
K35	Ia	light brown	graphite	80	2300	18	yellow
K37	Ia	light brown	graphite	77	1900	18	light brown
(1st run)
K37	Ia	light brown	sodium chloride	80	2400	12	yellow
(2nd run)
K38	Ia	medium brown	sodium chloride	77	1900	18	medium brown
(1st run)
K38	Ia	medium brown	sodium chloride	79	2050	18	light yellow
(2nd run)
K38	Ia	light yellow	99% NaCl +	79	2200	18	greenish-
(3rd run)			1% KNO3				golden yellow
K45	Ia	dark brown	graphite	80	2300	17	yellow
K43	Ia	light brown	sodium chloride	80	2200	18	light greenish
							yellow
K40	Ia	dark brown	96% NaCl +	80	2200	18	greenish-
			4% KNO3				golden yellow
K44	Ia	light brown	sodium chloride	80	2300	18	light yellow

[0054]

F1813	Ia	light brown	sodium chloride	80	2300	18	light yellow
(1st run)
F1813	Ia	light yellow	sodium chloride	80	2400	18	brownish
(2nd run)							yellow
F1814	Ia	very light	sodium chloride	80	2200	18	light yellow
(1st run)		brown
F1814	Ia	light yellow	sodium chloride	80	2400	18	orange
(2nd run)
F1816	Ia	light brown	sodium chloride	80	2400	18	orange
K56-1	IIa	light brown	sodium chloride	80	2300	18	colorless
K58-12	IaA/B	light brown	graphite	79	2300	18	deep yellow
K58-61	IaA/B	dark brown	graphite	79	2300	18	very light
							yellow
K58-62	IaA/B	dark brown	graphite	79	2300	18	colorless
K58-10	IaA/B	light brown	graphite	79	2000	18	colorless
K59-40	IaB	brown	graphite	79	2000	18	brown
K69-60	IaB	brown	graphite	79	2000	18	brown
K66-A	IIb	light steel	graphite	79	2300	18	light blue
		brown
K61-11B	IIa	medium brown	graphite	80	2300	18	light pink

[0055]

Claims

1. A method for treating colored natural diamond for changing its color, which comprises the following steps:

2. The method of claim 1, wherein said colored natural diamond has a brownish coloration.

3. The method of claim 1, wherein said colored natural diamond is Type IIa or IIb diamond.

4. The method of claim 3, wherein the diamond recovered from the press is colorless, pink, red, blue, or a color combination of thereof.

5. The method claim 1, wherein the diamond recovered from the press is colorless or a fancy colored diamond.

6. The method of claim 5, wherein said fancy colored diamond has a color selected from the group of pink, red, yellow, green, blue, purple, orange, and a color combination thereof.

7. The method of claim 1, wherein said elevated temperature ranges from about 1500ºto 2700ºC and said elevated pressure ranges from about 5 to 20 GPa.

8. The method of claim 7, wherein said HP/HT conditions are maintained for a time ranging from between about 30 seconds and 500 hours.

9. The method of claim 1, wherein said recovered diamond is subjected to step (b) a plurality of times.

10. The method of claim 1, wherein said pressure transmitting medium is thermally and chemically stable at HP/HT and is one or more of a salt, an oxide, or graphite.

11. The method of claim 10, wherein said pressure transmitting medium salt is one or more of chloride, iodide, or bromide of sodium, potassium, or calcium, or a mixture thereof.

12. The method of claim 10, wherein said pressure transmitting medium oxide is one or more of an oxide of magnesium, calcium, or mixtures thereof.

13. The method of claim 10, wherein said pressure transmitting medium is graphite.

14. The method of claim 1, wherein the colored natural diamond has a weight from about 0.1 to 100 carats.

15. The method of claim 1, wherein said colored natural diamond placed in said pressure transmitting medium is cut and polished.

16. The method of claim 1, wherein said recovered diamond is subsequently cut and polished.

17. The method of claim 1, wherein said recovered diamond has a Gemological Institute of America Clarity Grade of about IF, VVS, or VS.

18. The method of claim 16, wherein the cut is: Brilliant; Old Miners, Old European, Baguettes; Triangles; Emerald, Elliptical; or Free Form.

19. The method of claim 18, wherein said Old European comprises Perruzi and Mazarin; said Elliptical cuts comprise Pear, Heart, Oval, and Marquise; and said Free Forms comprise Scimitars, Briolletes, States, and Nations.

20. The method of claim 5, wherein the recovered diamond has a Gemological Institute of America color grade which is about D, E, F, G, H, I, J, K, L, M, N, O, or R.

21. The method of claim 6, wherein the recovered diamond has a Gemological Institute of America color grade which is about D, E, F, G, H, I, J, K, L, M, N, O, or R.