HIGH TEMPERATURE/HIGH PRESSURE COLOUR CHANGE OF DIAMOND

Abstract: A method is provided for changing the colour of a brown type IIa diamond from brown to colourless. The method involves subjecting the diamond to selected conditions of elevated temperature and elevated pressure 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, Ila and Mb 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 Ila 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 Ila diamond can exhibit a brown colour caused, it is believed, by structural deformation within the diamond crystal lattice 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 the colour of a brown type Ila diamond from brown to colourless 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 2200°C to 2600°C under a pressure of 7,6GPa to 9GPa for a suitable period of time The period of time during which the diamond is subjected to the conditions of step (II) will generally be longer the lower the temperature used in the range described above The period of heat treatment will generally not exceed 5 hours The preferred conditions for step (n) are a temperature in the range 2500°C to 2550°C for a period of up to 90 minutes Examples of suitable conditions for step (n) are 2500°C for a period of 1 hour 2300°C for a period of 5 hours The reaction mass may also be subjected to a two-stage heat treatment in step (II) In a first stage, the reaction mass is subjected to a temperature in the range 1900°C to 2300°C under a pressure of 6,9GPa to 8,5GPa for a suitable period of time and thereafter subjected to a higher temperature in the range described above In the first stage, the diamond colour is changed to a pink or a lighter brown and in the second stage the diamond colour is changed from pink or a lighter brown to colourless In this form of the invention, the first stage heat treatment will typically be for a period ranging from 10 minutes to 10 hours, preferably 20 minutes to 4 hours The preferred temperature is in the range 2100°C to 2300°C, and the preferred pressure is 7,4GPa to 8,5GPa The second stage heat treatment is preferably maintained for a period of up to 90 minutes and typically about 1 hour Longer times of up to 5 hours may be used, but are generally not desirable because of the high temperatures applied in this stage The preferred temperature is 2500°C to 2550°C and the preferred pressure is 8,4GPa to 8,6GPa, in the second stage DESCRIPTION OF EMBODIMENTS In the present invention, a brown type Ila diamond crystal, which will generally be natural diamond, is annealed under a pressure which prevents significant graphitisation in order to modify the structural deformation which gives rise to the brown colouration and thereby reduce the brown colour and produce a colourless diamond The two stage annealing process for etep (n) described above allows the colour change to occur in a controlled manner, limiting possible damage to the diamond The typical infrared absorption spectrum of a type Ila diamond is shown in Figure 1 However, it is not all type Ila diamonds which are suitable for colour enhancement by high temperature and high pressure treatment of the present invention The diamonds must have a brown colour which can vary from deep to light brown, including e g pinkish brown Those type Ila diamonds that are most suitable for the present invention have a nitrogen concentration of less than 2 ppm, preferably less than 0,2 ppm, exhibit a brown to pinkish brown colour and have a typical ultraviolet / visible absorption spectrum as shown in Figure 2 in graph line a, which shows a monotonically rising absorption, or a monotonically rising absorption with broad bands centred about 390 and 550 nm as shown in Figure 2 in graph line b The change in colour of the type Ila 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 such diamonds to the method of this invention results in a reduction or effective removal of both of the monotonically rising absorption and the absorption bands at 390nm and 550nm resulting in an effectively colourless diamond If the two stage heat treatment is used, there will be a reduction of strength of the monotonically rising absorption and the production or the enhancement of the strength of the broad absorption bands at 390 and 550 nm in the first stage 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 example EXAMPLE 1 A natural brown type Ila diamond, with the ultraviolet / visible absorption spectrum before treatment shown in Figure 3 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 4 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 co-operating metal 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 2250°C and a pressure of 7,8GPa which conditions were maintained for a period of 4 hours The contents of the capsule were thereafter subjected to a temperature of 2550°C and held at this temperature for 1 hour at a pressure of 8,6GPa 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 3 in graph line b The spectrum shows a significant reduction in absorption and the diamond can be regarded as colourless

Claims: CLAIMS 1 A method of changing the colour of a brown type Ila diamond from brown to colourless 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 2200°C to 2600°C under a pressure of 7,6GPa to 9GPa for a suitable period of time 2 A method according to claim 1 wherein the diamond is natural diamond 3 A method according to claim 1 or claim 2 wherein the diamond of type Ila has a nitrogen concentration of less than 2 ppm, exhibits brown to pinkish brown colour and has an ultraviolet / visible absorption spectrum which shows a monotonically rising absorption, or a monotonically rising absorption with broad bands centred about 390 and 550 nm 4 A method according to claim 3 wherein the nitrogen concentration is less than 0,2 ppm 5 A method according to any one of the preceding claims wherein the diamond, after being subjected to step (n), has a reduction or effective removal of both the monotonically rising absorption, and the absorption bands at 390 and 550 nm 6 A method according to any one of the preceding claims wherein the temperature in step (n) is in the range 2500°C to 2550°C A method according to any one of the preceding claims wherein the period during which the diamond is subjected to the conditions of step (II) does not exceed 5 hours A method according to any one of the preceding claims wherein the reaction mass in step (n) is subjected to a two-stage heat treatment, the reaction mass being subjected, in a first stage, to a temperature in the range 1900X to 2300°C and a pressure of 6,9GPa to 8,5GPa and the reaction thereafter being subjected, in a second stage, to a higher temperature in the range set out in claim 1 A method according to claim 8 wherein the temperature of heat treatment in the second stage is 2500°C to 2550°C and the period of heat treatment in this stage is up to 90 minutes 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 A method according to claim 1 and substantially as herein described with reference to the example

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, C30B29, C30B33