HIGH TEMPERATURE/HIGH PRESSURE COLOUR CHANGE OF DIAMOND

Abstract: The invention provides a method of changing a grey type IIb diamond from grey to blue or enhancing the colour of a type IIb diamond. Selected conditions of high temperature and high pressure are used to produce the colour change.

Description: HIGH TEMPERATURE/HIGH PRESSURE COLOUR CHANGE OF DIAMOND BACKGROUND OF THE INVENTION This invention relates to a method of changing the colour of diamond. Diamonds are generally classified into four main types: la, lb, lla and lib. These types are usually distinguished by infrared and ultraviolet spectra. Type la and lb diamonds contain nitrogen in a combination of different forms. Type lb contains single substitutional nitrogen or C-centres. la contains a combination of various nitrogen structures. Type lla diamonds have a nitrogen content of less than a few parts per million (ppm) and can be defined as those diamonds which show substantially no absorption in the 1332 - 400 cm "1 range when irradiated with infrared radiation. Type lla diamond can exhibit a brown colour caused, it is believed, by structural deformation within the diamond crystal lattice. Type lib diamonds have a nitrogen concentration of less than a few parts per million and a concentration of substitutional boron atoms which is greater than the concentration of nitrogen defects so as to give rise to uncompensated p-type semi-conductor behaviour. The absorption of the diamond in the visible region decreasing in strength with decreasing wavelength, gives rise to a blue colour. A combination of blue colour due to boron and brown colour due to structural deformation in type lib diamond leads to a bluish-grey to brownish-grey colour depending upon the relative strengths of the respective absorption. ?. - US 4, 124,690 describes a process for converting type lb nitrogen into type la nitrogen in type lb diamond by high temperature annealing under a pressure which prevents graphitisation. This treatment has the effect of reducing the yellow colour of the type lb diamond. SUMMARY OF THE INVENTION According to the present invention, there is provided a method of changing a grey type lib diamond from grey to blue or enhancing the colour of a type lib diamond which includes the steps of: (i) creating a reaction mass by providing the diamond in a pressure transmitting medium which completely encloses the diamond, and (ii) subjecting the reaction mass to a temperature in the range 1800°C to 2600°C under a pressure of 6,7GPa to 9GPa for a suitable period of time. Type lib diamonds have a nitrogen concentration of less than a few parts per million, and a concentration of substitutional boron atoms which is greater than the concentration of nitrogen defects so as to give rise to uncompensated p- type semi-conducting behaviour. A typical infrared, visible and ultraviolet absorption spectrum of type lib diamond is shown in Figure 1. The present invention has application to type lib diamond which is of a grey colour. The grey colour may range from grey to brownish grey, the spectrum of which exhibits an absorption monotonically increasing in strength with decreasing wavelength as shown in Figure 2 in graph line a. After the diamond has been subjected to the heat treatment of the invention, the blue colour is produced or enhanced and the diamond will have a spectrum which shows a reduction in absorption in the low wavelengths as shown in Figure 2 in graph line b. - J The blue colour in the diamond is enhanced by using an annealing temperature ranging from 1800°C to about 2600°C. Generally, the higher the annealing temperature the shorter the period of anneal. Generally, annealing times of more than 20 hours are not used. Preferably, the annealing temperature is 2200°C to 2500°C, under a pressure ranging from 6,7GPa to 9GPa, and preferably 7,6GPa to 8,4GPa, for a period ranging from 10 minutes to 15 hours, and preferably 1 hour. Examples of suitable annealing conditions are: 2500°C for 1 hour 2500°C for 10 minutes 2100°C for 20 hours 2300°C for 5 hours DESCRIPTION OF EMBODIMENTS In the present invention, a grey to brownish grey type lib diamond crystal, which will generally be natural diamond, is annealed under a pressure which prevents significant graphitisation in order to modify the structural deformation in the diamond which gives rise to the brown colouration, thereby reducing the brown colouration and enhancing the blue colour of the diamond. In some cases the initial boron concentration is so low that the absorption is dominated by a component causing the brown colour resulting in a brown diamond. Further, in some cases the boron is not detectable in absorption in the initial diamond probably due to an effect called charge compensation. It is to be understood that both such diamonds are included in the term "grey type lib diamond" as used herein and in the claims. Blue natural diamonds are rare. Thus, the present invention provides a method of enhancing the blue colour of a natural type lib diamond without causing damage to the diamond crystal. The change in colour of the type lib diamond crystal can be quantified by the changes in the absorption spectrum of the crystal taken before and after annealing. Spectra are taken of the crystal at room temperature by means of a spectrometer in a conventional manner showing the ultraviolet / visible absorption spectrum of the crystal. After the crystal is annealed, spectra are taken of it again at room temperature. Subjecting type lib diamond crystals to the annealing method of the invention results in a reduction of the strength of the monotonically rising absorption leaving dominant absorption at longer wavelengths so enhancing the blue colour. The annealing method of the invention also has the effect of increasing the concentration of uncompensated boron. This also contributes to enhancing the blue colour. In the method of the invention, a reaction mass is created by providing diamond in a pressure transmitting medium which completely encloses a diamond. The pressure transmitting medium is preferably a homogeneous pressure transmitting medium which evenly spreads the pressure which is applied over the entire surface of the diamond which is being treated. Examples of suitable mediums are those which have a low shear strength such as metal halide salts. Examples of suitable metal halide salts are potassium bromide, sodium chloride, potassium chloride, caesium chloride, caesium bromide, copper chloride and copper bromide. Such mediums have been found to provide the desirable even spread of pressure which ensures that any graphitisation which may occur on the diamond surface is kept to a minimum. A particular advantage of using a metal halide salt as the pressure transmitting medium is that the diamonds can be easily recovered after treatment by dissolving the medium in hot water. The method of the invention may be used to treat a single diamond or a plurality of discrete diamonds. If a plurality of discrete diamonds are treated simultaneously, each diamond should be separated from its neighbours by the pressure transmitting medium. The maximum volume of diamond which may be treated is limited only by the capacity of the high pressure/high temperature apparatus used. Conventional high temperature/high pressure apparatus may be used in the method of the invention. Various reaction vessel configurations which provide for indirect or direct heating of the reaction mass are disclosed in the patent literature and are useful in carrying out the present annealing process. These reaction vessels usually consist of a plurality of interfiling cylindrical members and end plugs or discs for containing the reaction mass in the centremost cylinder. In the indirectly heated type of reaction vessel one of the cylindrical members is made of graphite which is heated by the passage of electric current therethrough and which thereby heats the reaction mass. In the directly heated type of reaction vessel, the reaction mass is electrically conductive, thereby eliminating the need for an electrically conductive graphite cylinder, and electric current is passed directly through the reaction mass to heat it. The invention is illustrated by the following examples. EXAMPLE 1 A natural brownish grey type lib diamond, with the ultraviolet / visible absorption spectrum before treatment shown in Figure 2 in graph line a, was used. A plurality of such diamonds were placed in a pressure transmitting medium in a reaction vessel of the type illustrated by Figure 3. Referring to this figure, the diamond crystals 10 are placed in a pressure transmitting medium 12 such that the crystals are discrete and separated from each other in the pressure transmitting medium. The diamonds are preferably evenly dispersed in the medium. The pressure transmitting medium is preferably a low shear strength medium of the type described above. The diamond- containing medium 12 is placed in a container 14 made of graphite, pyrophyllite, magnesium oxide, or zirconium oxide, which is enclosed by cooperating cups 16, 18 which form a metal canister around the container 14. The metal may be molybdenum, tantalum or steel. The canister may be compacted around the container to eliminate air voids. The loaded canister is now placed in the reaction zone of a conventional high temperature/high pressure apparatus. The contents of the capsule were subjected to a temperature of 2200°C and a pressure of 7,6GPa which conditions were maintained for a period of 12 hours. The capsule was removed from the apparatus and allowed to cool. The canister and graphite container were removed and the diamonds recovered from the medium. The ultraviolet / visible absorption spectrum of the diamond after treatment is shown in Figure 2 in graph line b. The spectrum shows the reduction in the brown background after annealing. As the absorption spectrum is now dominated by an absorption in the red end of the spectrum, decreasing with decreasing wavelength, the diamond appears blue. EXAMPLE 2 A natural brownish grey type lib diamond was treated in the manner generally set out in Example 1 , save that the annealing temperature was 2500°C at the same pressure indicated in Example 1 , and these conditions were maintained for a period of 1 hour. The ultraviolet / visible absorption spectrum before treatment is shown in Figure 4 in graph line a, whilst the ultraviolet / visible absorption spectrum of the diamond after treatment is shown in Figure 4 in graph line b. The spectrum again shows a reduction in the brown background after annealing. Further, the increased adsorption in the red part of the visible spectrum so deepening the blue colour is caused, in part, by an increase in the concentration of uncompensated boron.

Claims: CLAIMS 1. A method of changing a grey type lib diamond from grey to blue or enhancing the colour of a type lib diamond includes the steps of: (i) creating a reaction mass by providing the diamond in a pressure transmitting medium which completely encloses the diamond, and (ii) subjecting the reaction mass to a temperature in the range 1800°C to 2600°C under a pressure of 6,7GPa to 9GPa for a suitable period of time. 2. A method according to claim 1 wherein the period of heat treatment in step (ii) does not exceed 20 hours. 3. A method according to claim 1 or claim 2 wherein the conditions of step (ϊή are 2000°C to 2500°C under a pressure ranging from 6,7GPa to 9GPa for a period ranging from 10 minutes to 15 hours. 4. A method according to any one of the preceding claims wherein a plurality of diamonds is placed in the pressure transmitting medium, each diamond being separated from its neighbour by pressure transmitting medium. 5. A method according to claim 1 and substantially as herein described with reference to the Examples.

Inventors: BURNS ROBERT CHARLES, FISHER DAVID, SPITS RAYMOND ANTHONY, BURNS, ROBERT, CHARLES, FISHER, DAVID, SPITS, RAYMOND, ANTHONY
Applicants: BURNS ROBERT CHARLES, DE BEERS IND DIAMOND, FISHER DAVID, SPITS RAYMOND ANTHONY, BURNS, ROBERT, CHARLES, DE BEERS INDUSTRIAL DIAMONDS (PROPRIETARY) LIMITED, FISHER, DAVID, SPITS, RAYMOND, ANTHONY
Application year: 2001
Main classification: B01J3
IPC codes: B01J3, C01B31