CA1112134A - Annealing natural or synthetic diamond crystal - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Type Ib diamond crystal is annealed at an annealing temperature ranging from about 1500°C to about 2200°C under a pressure which prevents significant graphitization of the diamond during the annealing to convert at least about 20% of the total amount of type Ib nitrogen present in the crystal to type Ia nitrogen.
Where the diamond crystal is of gem quality the lighter color associated with the type Ia crystal isusually preferred. The type Ia crystal is also harder and stronger for use as an abrasive. The crystal norphology is unchanged in the annealing process. Annealing of synthetic crystal results in novel type Ia crystal forms not found in nature such as the cubo-octahedral form, particularly preferred for use in diamond saw blades.

Description

~ 4 RD 9039 This invention relates to the annealing of diamond type Ib to convert at least a portion of it to type Ia diamond.
Diamonds are generally classified into four main types: Ia, Ib, IIa, and IIb. These types are most easily distinguished by infrared and ultraviolet spectra and sometimes by electron paramagnetic resonance (EPR). Type Ia and Ib diamonds contain dissolved nitrogen; in Ia diamonds, most of the nitrogen is not EPR active and appears to be in aggregated form;
in Ib diamonds most of the nitrogen is EPR active, and is atomically dispersed. Types IIa and IIb diamonds do not contain appreciable nitrogen. Each type of diamond has typical infrared and ultraviolet spectra with characteristic features.
The large majority of synthesized diamonds are type Ib, but type IIa diamonds can easily be made either by excluding nitrogen from the diamond growing media or by using appropriate nitrog~n getters.
The large majority of natural diamonds examined are type Ia. No type Ia diamonds have been synthesized thus far in the laboratory. Natural type Ia diamond crystals can have a variety of colors, with many being pale yellow to colorless. Such a diamond crystal can also be a combination of pale yellow and colorless areas as well as exhibit local variations in its characteristic color in different parts of the crystal. Ordinarily, it has a rounded dodecahedral or octahedral morphology. `
There are two forms o~ type la diamond, an A
band form and a B band form, and these forms are dis-tinguishable by their infrared, visible and ultraviolet absorption spectra. Usually, however, these two forms are most easily differentiated by their infrared spectra wherein the A band form has its main absorption band coming at 1280 cm~l and the B band form hes its main absorption band coming at 1175 cm~l. While each form of type Ia appears to be thermodynamically more stable than type Ib diamond, the present process has thus far produced only the A band form which is hereinafter referred to broadly as type Ia.
Synthetic diamonds are substantially the same as natural diamonds but there ~re enough di~erences between them to distinguish betweell the natural and synthetic crystals. These differences are mainly in morphology, surface appearance, impurity inclusions and the nature of impurity imperfections 9uch as the different forms of nitrogen. As found, natural diamond crystals most frequently have curved edges and convex faces. On the other hand, syntheSized diamond crystals, as grown, have sharp edges, flat and relatively smooth ~aces.

2i34 ~

Depending on the conditions of growth, synthetic type Ib crystals ha~e octahedral or cubo-octahedral morphology, the latter sometimes having small (113) faces. Also, depending on the conditions of growth, a synthetic type Ib crystal may have no definite morphology and can have a wide variety of shapes with some substantially distorted from the octahedral or cubo-octahedral regular shapes, as well as in extreme cases, highly irregular particles of no particular shape, Impurity lnclusions in synthetic diamonds are metal catalysts whereas in natural diamonds they are a variety of minerals, and these impurity inclusions are detectable by several techniques such as electron diffraction analysis or X-ray analysis.
Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, con9idered in con~unction with the figures accompanying and forming a part of the specification in which:
Figure 1 represents the phase diagram of carbon showing the diamond-graphite equilibrium line and the shaded area defines the Region of Conversion which encompasses the required annealing temperatures and the corresponding annealing presRures of the present process.
Figure 2 is a sectional view of a preferred reaction vessel for carrying out the present invention.

~ ` Rr~ ~039 Figure 3 shows a typical infrared absorption spectrum of the region of interest for a type Ia natural diamond crystal.
Figure 4 shows a typical infrared absorption spectrum of the region of interest for a synthetic type Ib diamond crystal.
Figure 5 shows infrared absorption spectra of the region of interest of a synthetic type Ib crystal taken before and after it was annealed in accordance with this invention.
Figure 6 shows infrared absorption spectra of a natural mixed type Ib-Ia diamond crystal before and after annealing in accordance with this invention.
Briefly stated, the present invention as it relates to a process comprises annealing type Ib diamond crystal at an annealing temperature ranging from about 1500C to about 2200C under a pressure which prevents significant graphitization of the diamond during annealing to convert at least about 20%
of the total amount of type Ib nitrogen present in the crystal to typ~s Ia nitrogen. As it relates to a product the present: invention comprises diamond crystal of type Ia of novel morphological form as prepared from synthetic diamond crystal.
In the present process the amount of conversion of type Ib nitrogen to type Ia nitrogen is determinable by a number of conventional techniques. The most frequently used technique is one where it is revealed by the differences or changes in the absorption spectra of the Ib crystal taken before and after annealing. Speci-fically, spectra are taken of the type Ib crystal ~$~ 34 at room temperature by means of spectrometers in a conventional manner showing the ultraviolet, visible and infrared absorption spectra of the crystal. After the cyrstal is annealed, spectra are taken of it again at room temperature showing its ultraviolet, visible and infrared absorption spectra. From a comparison of the changes in these spectra, the amount of conver-sion of type Ib nitrogen to type Ia nitrogen, i.e.
the percent of the total amount of type Ib nitrogen present in the crystal converted to type Ia nitrogen, is determinable in a conventional manner.
Generally a type Ib diamond crystal has a color depending on the amount of nitrogen dissolved in the crystal. The color of the crystal ranges from a green to a greenish-yellow to a yellow with the maximum or largest amount of dissolved nitrogen producing the greenish-yellow color. Likewise, the amount of nitrogen dissolved in the crystal determines the intensity of the yellow color whlch can range from a deep golden yellow to a pale yellow with the deep golden yellow indicating substantially more dissolved nitrogen than the pale yellow. In addition, the Ib diamond cyrstal can exhibit a mixture of greenish-yellow and/or yellow colors or shades, i.e. it can exhibit local variations in its characteristic color and intensity, which indicates regions of varying nitrogen content.

~ RD-9039 3~

In the present process there is no limitation on the size of the type Ib diamond crystal. Specifically, the minimum size of the crystal can be one micron or less and the maximum size is limited only by the capacity of the annealing eqùipment. For most present applications, the Ib crystal size ranges from about 0.25 millimeter to about 6 millimeters. The size of the diamond crystal given herein is that measured along the longest edge dimension of the crystal.
The present annealing process is carried out in high temperature-high pressure apparatus normally used for synthesizing diamonds by application of high temperatures and pressures to a suitable reaction mass or specimen.
One preferred form of a high pressure-high ~mperature apparatus in which the present invention can be carried out is disclosed in United States Patent No. 2,941,248 issued June 21, 1960 to Howard T. Hall and assigned to the present assignee, and it is also disclosed in numerous other patents and publications.
Those skilled in the art are well acquainted with this "belt-type" apparatus and, for this reason, the apparatus is not illustrated. Essentially, the apparatus consists of a pair of cemented tungsten carbide punches disposed at either side of an intermediate belt or die member of the same material. The space between the two punches and the die is occupied by the reaction vessel and surrounding gasket/insulation assemblies therefor. High pressures are generated in the reaction vessel from the compressive forces caused by the relative movement of the co-axially disposed punches toward each other within the die, Means are provided for heating the reaction mass in the reaction vessel during the application of pressure, There are, of course, various other apparatuses capable of providlng the required pressures and temperatures that can be employed within the scope of this inv'ention such as tetrahedral types, cubic types and spherical types, Operational techniques for applying high pressures and temperatures in the apparatuses useful in the present process are well known to those skilled in the super-pressure art.
Various reaction vessel configurations whlchprovide 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 o inter-fitting cylindrical members and end plugs or disc~ for containing the reaction mass in the centermost cylinder, In the indirectly heated type of reaction vessel one of the cylindrical members i9 made of graphite which is heated by the passage of electric current therethrough ~7--, . . .. ., . -: . ; . . - . , ., . :~
: . ..
:
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_ RD-9039 12~4 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 above-referenced United States Patent No. 2,941,248 of Hall di~closed an embodiment of a reaction vessel wherein the reaction speciment is indirectly heated, as well as the alternative embodiment for directly heating the reaction specimen when it is electrically conductive.
United Sta~es Patent No. 3,031,269 issued April 24, 1962 to Harold P. Bovenkerk and assigned to the present assignee, discloses a reaction vessel for indirect heating of the reaction mass. Specifically, the outer element of the reaction ve8sel is a hollow pyrophyllite cylinder. Positioned concen-trically within and adjacent to the pyrophyllite cylinder is a graphite electrical resistance heater tube.
Within the graphite tube there is concentrically positioned an alumina cylinder which holds the reaction mass or specimen.
The directly heated embodiment of the reaction vessel is preferred in the present process, and a particu-larly preferred form is shown in Figure 2. Specifically, this reaction vessel includes a hollow outer cylinder 3 RD^9039 made of non-conducting material such as pyrophyllite.
Positioned concentrically within and ad~acent pyrophllite cylinder 3 is ceramic cylinder 4 preferably made of alumina. Charge element or insert assembly 5 is adapted to fit concentrically in ceramic cylinder 4 and is dimensioned for a close fit with cylinder 4. Charge element 5 is comprised of graphite rod 7 and graph~te rod 6 wherein the graphite is of spectroscopic purity, Graphite rot 6 has hole 8 which i9 drilled to fit closely around diamond cry~tal 9, i.e., the diamond to be annealed, Diamond crystal 9 should not pro~ect outside of hole 8 since such projection would prevent a close contiguous fit of rod 7 with rod 6, Rod 7 should be in electrlcal contact with rod 6 at surface 10, Preferably, the top 9urface of diamond 9 is flush with surface 10 of graphite rod 6. Any space between di~mond crystal 9 and hole 8 is preferably ~illed with an electrlcally conducting material, such as graphite powder of spectro-scopic purity, to promote passage of the electric current and thereby promote heatin8 of diamond 9. Electrically conducting circular metallic discs 1 snd 2 close the ends of graphite rods 6, 7, and cylinters 3 and 4 ~iscs 1 and 2 are preferably made of a metal such as nickel or tantalum and must be in electrlcal contact with graphite rods 7 and 6, respectively. Since graphite _9_ : . ... . , .. - ., ~

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2~3 4 rod 6 is electr1cally conducting and diamond crystal 9 is not electrically conducting, the highest temperatures are attained and maintained at the thinnest portions of graphite rod 6, e.g., the area of graphite rod 6 Qurrounding diamond crystal 9, After assembly of the reaction vessel and introduction thereof into the high pressure high temperature apparatus within the gasket/insulation assemblies, preferably pressure is raised first and then the ,temperature. The rates of increase of pressure or temperature are not critical. When pressure and temperature are at a level in the Region of Conversion defined in Figure 1, they are held at that level for a period of time sufficient to attain the desired conversion of at least about 20% of the diamond'9Ib type nitrogen to type Ia nitrogen. When the de~ired con-ver8ion i9 attained, the electrical power which heats the diamond crystal i8 shut off and the crystal cools to about room temperature quickly, usually in about L minute. Generally, when the crystal has cooled to below 50C, the pressure is then released, preferably at a rate of about 10 kilobars per minute to atmospheric pressure, In the present annealing process a type Ib diamond is annealed at a temperature ranging from .. : . ...... . . . . ..

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about L500QC to about 2200C. Annealing temperatures lower than about 1500C are not operable or take too long a period of annealing time to be practical. Annealing temperatures higher than 2200C provide no significant advantage. Anneal-ing temperatures ranging from 1600C to 2000C are preferred since they are not too difficult to attain, do not require excessively high pressures and since they induce high rates of conversion.
The pressure used in the present process need only L0 be 9u~ficient to maintain the diamond stable at the annealing temperature. Specifically, it is a pressure which prevents graphitization or prevents significant graphitization of the diamond crystal at the annealing temperature. The shaded area of Figure 1 defines the Region of Conversion which defines the operable temperatures and corresponding annealing operable pressures of the present proces9. The tiamond-~raphite equilibrium line as well as pressure and temperature cali~
brations at such superpressures are not definitely known.
The diamond-graphite equilibrium line shown in Figure 1 is the best approximation known at present for diamond-graphite equilibrium. Preferably, the present process is carried out at or above this diamond-graphite equilibrium line. The shaded area in Figure 1 of the Region of Con~ersion below the diamond-graphite equilibrium line i9 a tolerance zone which shows the lower pressures which are operable in the present process for limited periods of ~ime. For example, for 2 ~3 ~

the minimum pressures shown by the tolerance zone, the maximum period of annealing time is about one hour without significant graphitization of the diamond crystal occurring. If annealing times longer than one hour are used, then the pressure applied in the tolerance zone should be closer to the diamond-graphite equilibrium line, As shown in Figure 1 by the Region of Conversion, an annealing temperature of about 1500C requires a pressure of at least about 48 kilobars, at 1600C the pre8sure should be at Least about 51 kilobars and preferably about 61 kilobars, at 2000C the pressure should be at least about 63 kilobars and preferably about 74 kilobars, and at a temperature of about 2200C the pressure should be at least about 70 kilobars and preferably about 80 kilobars, Annealing time, i.e. the period of time at annealing temperature and pressure, i9 determinable empirically and can range from about one minute to about 50 hours, and preferably up to about 20 hours. U~ually it range9 from about 10 minutes to about 5 hours. Speci-~ically, annealing time depends largely on annealing temperature, the kind of Ib crystal being annealed as determined by its nitrogen content, and the extent or degree of conversion of the type Ib to Ia required. With rising annealing temperatures, the rate of conversion of ..

,- RD 9039 type Ib to type Ia increases significantly, i.e. more than five times in going from 1600C to 2200C.
The mechanism of the present process is not understood but it is believed that the rate of conversion of Ib nitrogen to Ia nitrogen does not differ significantly between a crystal of high nitrogen content and one of low nitrogen content, but the period of annealing time at a given annealing temperature to leave essen-tially the same amount of type Ib nitrogen in each crystal does differ since the Ib crystal with the higher nitrogen content has more nitrogen to convert to type Ia thereby requiring a longer annealing time.
While the detailed mechanism of the conversion process i8 not understood, annealing experiments have shown that the activation energy for the process is approximately 83 kilocalories/mole t3.6 eV).
The extent of conversion of a Ib crystal to Ia is determinable empirically by a number o~
known methods in the art. For example most of the nitrogen in type Ia crystal is EPR inactive tE1ectron Paramagnetic Resonance) whereas the dissolved nitrogen in type Ib diamond crystal is EPR active. Also, types Ia and Ib crystals each have typical infrared, visible and ultraviolet spectra with characteristic features which are identifiable in infrared, visible and ultra-violet spectra of a crystal of mixed type Ia and Ib.
Preferably, to determin satisfactory annealing times and temperatures for a particular kind of type Ib crystal, e.g., a crystal containing a certain amount of 34 : ~
dissolved nitrogen as reflected by its infrared, visible and ultraviolet spectra and the intensity of its color, the type Ib crystal should preferably be initially produced in the form of a platelet polished on both sides so that the spectra taken thereof are well-defined. The platelet is then annealed at a given annealing temperature for a certain period of time and after each annealing run, its infrared, visible and ultraviolet spectra are taken. A comparison of spectra taken before and after annealing indicates the extent of conversion to type Ia. Also, additional comparisons of such spectra with EPR spectra of the type Ib crystal before and after annealing are another indication of the extent of conversion to type Ia. Once the time for annealing this particular kind of type Ib crystal has been determined to attain a certain conversion to type Ia, such annealing time and annealing temperature can be used for the same kind of Ib crystal, e.g. a crystal containing substantially the same amount of dissolved nitrogen, regardless of its size or shape, to attain the same or substantially the same degree of conversion to type Ia.
Also, after the reaction rates are determined by experiments on a particular kind of Ib crystal, it is possible to estimate the correct annealing times which would leave a specified amount of type Ib nitrogen in the crystal for type Ib crystals having a wide range of nitrogen concentrations initially.
In the present process from at least about 20%
up to about 100% of the total amount of type Ib nitrogen present in the crystal is converted to type Ia nitrogen .
:

~ 4 RD 9039 However, regardless of annealing conditions a residue of type Ib nitrogen in an amount of less than 1~ of the total nitrogen present in the crystal will always remain in the crystal and such type Ib nitrogen residue can be as low as 0.001%, or lower, of the total amount of nitrogen present in the crystal. Initial conversion to type Ia nitrogen lower than 20% of the total amount of type Ib nitrogen present in the crystal may not effect the physical pro-perties of the crystal significantly for most applications.
The extent or degree of conversion of type Ib nitrogen to type Ia nitrogen depends largely on the particular properties desired. In the annealed crystal produced by the present process which contains both types Ia and Ib, type Ia appears to be uniformly distributed throughout type Ib.
A a result of the present process, at least a portion of the type Ib crystal undergoes some change in color or shade, i.e. in a greenish-yellow crystal at least a portion changes toward the yellow or for a yellow crystal a portion becomes at least a shade lighter yellow, the extent of which depends on the extent of its conversion to type Ia. Also, when substantially all or all of the type Ib nitrogen is converted to type Ia nitrogen, the result is a very pale yellow and/or a colorless cyrstal which has many uses as jewelry, and which frequently is of gem quality.
The annealed diamond crystals produced by the present process are useful as abrasives. The abrasive industry requires numerous types of abrasive materials to carry out various grinding or machining operations, the requirements of which are determined largely by the properties of the material being machined and, to some ;34 extent, the results desired. For certain operations in the abrasive industry, synthetic type Ib crystal has been satisfactory and for other operations natural type Ia crystal has been satisfactory. However, as a result of the present invention, the abrasive industry now has available a crystal which is a mixture of types Ib and Ia, the composition of which can be controlled to produce crystals with graded physical properties over a wide range to adjust the crystal to the particular abrasive use to which it is applied. Specifically, ; with increasing degrees of conversion of type Ib to type Ia, the crystal changes in abrasive properties, usually becoming harder and stronger. As a result, a mixed type Ib-Ia crystal can be produced having optimum properties for a particular abrasive use.
When substantially all or all of the type Ib nitrogen in the crystal is converted in the present process, the resulting type Ia crystal is also highly useful as an abrasive.
The annealed diamond cry~tal~ of the present process are also u3eful as jewelry, especially those of gem quality.
The morphology of the as-grown synthetic type Ib diamond crystal is retained in the present process whether thy be regular octahedral, cubo-octahedral or those shapeless forms which are specifically grown for abrasive wheel use. The latter type crystals are never found in nature. However, the preferred as-grown type Ib crystal has a cubo-octahedral morphology which is particularly useful in diamond saws. Also, the as-grown type Ib crystal may have an octahedral morphology.

2~34 In such instance where the present annealed crystal, polished or unpolished, has a shape which does not reveal it to be synthetic, it can be identified as a synthetic type Ia containing crystal by a known light scattering technique. Specifically, this technique comprises examining the crystal under a micro-scope by shining a beam of light at an angle thereon and observing the scattered light reflected from scattering centers normally present in synthetic diamond, but such scattering centers, and resulting scattered light, are not known to have been seen in natural type Ia diamond crystal.
The invention is further illustrated by the following examples which are tabulated in Table I
and wherein the procedure was as follows unless otherwise stated:
Type Ib synthetic diamond crystals were pr#pared in a high pressure-high temperature apparatus of the "belt-type" disclosed in U. S. Patent No.
2,941,248 - Hall - dated June 21, 1960.
In each example the diamond crystal was at least partly polished in a conventional manner using a scaife prior to annealing. The resulting plate had a significantly uni~orm thickness which ranged from about 1/2 mm to about 1 mm. The size of the plate given in Table I is its maximum width.
Each diamond crystal was annealed in a reaction vessel as shown in Figure 2. Graphite rods 6 and 7 were of spectroscopic purity and of the same size, each was 80 mils in diameter and 225 mils in length. A hole 8 was drilled in rod 6 to a size to fit closely around the particular diamond being annealed . .' ' : , :
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~ 34 ~D 9039 and any surface between the diamond crystal and inner sur-face of hole 8 was filled with graphite powder of spectro-scopic purity. In all of the examples the diamond crystal did not protrude from the drilled hole 8 and electrical contact between rods 6 and 7 was maintained as shown in Figure 2. Ceramic cylinder 4 was made of alumina and had an inner diameter of about 80 mils and a wall thickness of 60 mils. Cylinder 3 was made of pyrophyllite and had an inner diameter of about 200 mils and a wall thickness of 75 mils. Metallic disc members l and 2 were circular, of the same size, each with a diameter of 350 mils and a thickness of 10 mils, and made of tantalum. The discs were in electrical contact with rods 6 and 7 as shown in Figure 2.
To carry out the present annealing process, this reaction vessel was use.d in the "belt-type" apparatus disclosed in U. S. Patent No. 2,948,248 - Hall.
Absorption spectra ranging from the ultraviolet through the infrared were made of the diamond crystals at room temperature before and after they were annealed.
Electron paramagnetic re80nance (EPR) spectra were made of the diamond crystals at room temperature before and after the crystals were annealed.
With respect to infrared spectra measurements, although the 1130 cm 1 band in type Ib crystals is normally used to characterize the Ib crystal, in the present instance, for purposes of accuracy, the 1345 cm 1 band which is cor-related to type Ib nitrogen was used to determined the con-version of the type Ib nitrogen to type Ia nitrogen.
In the table examples 1 and 2 refer to synthetic diamond crystal preferred as previously mentioned, and example 3 refers to a mixed type Ib-Ia natural crystal.

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le Z r) _ _ ~3 4 Table I illustrates the present invention, The - decrease in EPR and infrared intensities in the type Ib diamonds of Table I was used to monitor the conversion of type Ib nitrogen to type Ia nitrogen. The alternatives that a decrease in these type Ib diæmond EPR and in~rared intensities could also occur if the Ib nitrogen was diffusing out of the crystal with no conversion to type Ia nitrogen or changing to nitrogen of yet another type were ruled out for two reasons. The first reason is qualitative L0 ln that type Ia infrared absorption bands do appear, he~ce there is some conversion to type Ia nitrogen, The second reason is quantitative in that from the type Ib and/or type la absorption bands present one can calculate, based on published data, using standard techniques, the total amount of nitrogen present. For the present annealing experiments this nitrogen content of any one diamond remains constant, within experimental error, For instance, Example 1 had an initial content of only type Ib nitrogen of 365 ppm, After the annealing treatment, the infrared absorption spectra and EPR for Examples 1 and lA
showed that the type Ib nitrogen decreased to 35 percent and 39 percent, respectively, of its original content as shown in Table I, yet the total nitrogen was 364 ppm, the same as before the annealing process. Hence, no change occurred in the total nitrogen content despite the fact that the final type Ib nitrogen was approximately '' ,-- RD 9039 35 to 39 percent of that originally present. From the IR and EPR spectra of examples 3 and 3A the decrease in type Ib nitrogen content is estimated as 45 and 55 percent respectively, although the total nitrogen was unchanged in the annealing process, being about 135 ppm.
Therefore, the change in intensity of the type Ib absorption band at 1343 cm 1 is a good indication that type Ib nitrogen is being converted to type Ia nitrogen and is not diffusing out of the diamond or being converted to nitrogen of yet another type.
It is understood that the present annealing process can be carried out with the same diamond crystal more than one time to additionally increase the amount of type Ia nitrogen therein. For example, a mixed type Ib-Ia annealed diamond crystal produced by the present process can be annealed in accordance with the present process to convert an additional amount of type Ib nitrogen to type Ia nitrogen.

Claims (14)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. An annealing process for converting type Ib nitrogen in diamond crystal to type Ia nitrogen, said crystal having a minimum size of one micron as measured along the longest edge dimension of the crystal, which consists essentially of subjecting said diamond crystal to an annealing temperature ranging from about 1500°C to about 2200°C under at least a pressure which prevents significant graphitization of said crystal at said annealing temperature for a period of time ranging from about one minute to about 50 hours and sufficient to convert at least about 20% of the total amount of type Ib nitrogen present in said crystal to type Ia nitrogen, the minimum pressure ranging from 48 kilobars at said annealing temperature of 1500°C to a minimum pressure of 70 kilobars at said annealing temperature of 2200°C, and recovering the resulting annealed free diamond crystal, wherein the dissolved nitrogen detectable by ultraviolet and visible and infrared absorption spectra and electron paramagnetic resonance consists of types Ib and Ia nitrogen, said type Ib nitrogen always being present in said annealed crystal in at least a detectable amount, said annealing process making no significant change in the morphology and physical dimensions of said diamond crystal.

2. An annealing process according to claim 1, wherein said annealing temperature ranges from about 1600°C to about 2000°C, and said minimum pressure ranges from 51 kilobars at 1600°C to a minimum pressure of 63 kilobars at 2000°C.

3. An annealing process according to claim 2, wherein said annealing temperature is about 1900°C.

4. An annealing process according to claim 1, wherein at least about 50% of the total amount of type Ib nitrogen present in the crystal is converted to type Ia nitrogen.

5. An annealing process according to claim 1, 2 or 3, wherein the resulting annealed crystal is mixed type Ib-Ia which is further annealed in accordance with the process of claim 1 to convert an additional amount of type Ib nitrogen to type Ia nitrogen.

6. An annealing process according to claim 1, 2 or 3, wherein said crystal is synthetic.

7. An annealing process according to claim 1, 2 or 3, wherein said crystal is natural.

8. An annealing process according to claim 1, 2 or 3, wherein said crystal is natural and comprises a mixture of type Ib and Ia crystal prior to said converting.

9. A free annealed synthetically grown diamond crystal wherein the morphology of said crystal is substantially as-synthetically grown, said as-synthetically grown morphology consisting essentially of sharp edges and flat faces, said crystal having nitrogen dissolved therein detectable by ultra-violet, visible and infrared absorption spectra and electron paramagnetic resonance, said dissolved nitrogen consisting of types Ia and Ib with said type Ib nitrogen always being present in said crystal in at least a detectable amount, and wherein at least about 20% of the total amount of said detectable nitrogen present in said crystal is type Ia, said crystal having a minimum size as measured along the longest edge dimension of the crystal of one micron, said free annealed synthetically grown diamond crystal being produced by the process of claim 1.

10. An annealed synthetically grown diamond crystal according to claim 9, wherein at least about 50% of the total amount of said detectable nitrogen present in the crystal is type Ia.

11. An annealed synthetically grown diamond crystal according to claim 9, wherein more than 99% of the total amount of said detectable nitrogen present in the crystal is type Ia.

12. An annealed synthetically grown diamond crystal according to claim 9, wherein said crystal has an octahedral morphology.

13. An annealed synthetically grown diamond crystal according to claim 9, wherein said crystal has a cubo-octahedral morphology.

14. An annealed synthetically grown diamond crystal according to claim 9, wherein said crystal contains an inclusion of a diamond-growing metal catalyst.