WO2018122552A1

WO2018122552A1 - Identification of mounted gemstones

Info

Publication number: WO2018122552A1
Application number: PCT/GB2017/053867
Authority: WO
Inventors: Colin MCGUINNESS; Peter Lanigan
Original assignee: De Beers Uk Limited
Priority date: 2016-12-28
Filing date: 2017-12-21
Publication date: 2018-07-05
Also published as: GB201622314D0

Abstract

There is disclosed a method of identifying a mounted diamond as natural or synthetic. The diamond is irradiated with at least one excitation pulse of ultra-violet radiation. During and/or following the excitation pulse, light emitted by the diamond in at least one time window having a predetermined time relationship relative to the excitation pulse is detected so as to obtain luminescence data. The or each time window is chosen to include luminescence having a decay time characteristic of one or more one or more specific markers in the luminescence properties of the diamond. The method further comprises processing the luminescence data to remove light emitted at wavelengths shorter than a specified wavelength.

Description

IDENTIFICATION OF MOUNTED GEMSTONES

Technical Field The present invention relates to a method of identifying mounted gemstones. In particular, although not exclusively, the invention relates to a method and an apparatus for identifying mounted diamonds, such as diamonds mounted in watches.

Background

Synthetic or man-made diamonds, manufactured by HPHT (high pressure high temperature), CVD (chemical vapour deposition) or other industrial, non-geological processes, have a wide variety of industrial applications, but currently form only a small percentage of the gemstone industry. Being man-made, they do not attract the high values associated with natural diamonds of similar colour and quality and it is clearly desirable from a consumer perspective to provide reliable means of identifying and separating synthetic diamonds from natural ones. This identification is especially useful when valuing objects such as jewellery and watches, which may have gemstones mounted therein.

One characteristic that has proven to be useful in the identification of diamonds is the emission of luminescence when a diamond is illuminated (or excited) by a source of energy, most commonly but not exclusively, electromagnetic radiation. A gemmologist would normally have an ultraviolet lamp, perhaps emitting radiation with a wavelength of 365nm or 254nm (nanometres), these being common lines in the emission of the low pressure mercury lamp, and might observe what would be called fluorescence. Fluorescence is a type of luminescence characterised as only being produced when the ultraviolet excitation is on. Phosphorescence, which may also be observed, is a type of luminescence that remains but decays away once the excitation is removed.

The use of specific markers in the luminescence properties of a gemstone for identification purposes is described in PCT/GB2016/051927. A UV source, producing electromagnetic radiation at wavelengths of substantially 225 nanometres (nm) or less, is used to illuminate (i.e. excite) a diamond sample with a series of pulses of UV light. The UV source is synchronised with an image capture device, such that they are both triggered simultaneously. The image capture device may be configured with a delay, however, so that image capture does not begin until after the UV excitation pulse has ended. In this way, only phosphorescence is captured and any fluorescence produced during excitation is excluded or "gated out". The end of the image capture may also be carefully selected so that only luminescence produced during a specific time window is captured. The diamond may be repeatedly irradiated so that multiple luminescence images are captured. These multiple captures may then be combined by a processor to create an image of the diamond that is suitable for visual analysis. The luminescence data so produced can be analysed in order to establish the presence or absence of one or more specific markers. For example, one of the markers may be a blue fast phosphorescence marker, which comprises luminescence in a wavelength band peaking at about 450 nm and with a decay of less than about 80 milliseconds (ms). Testing for this marker may involve testing, in a time window opening at or after the end of the excitation pulse and ending about 80 ms after the end of the associated excitation pulse, for a luminescence band peaking at about 450 nm. The presence of the blue fast phosphorescence marker may be an indicator that the diamond under test is a natural type lla or la diamond. The method described above may be used in the identification of both mounted and loose gemstones. However, direct irradiation of the surface of a mounted gemstone is not always possible. This may occur, for example, where a gemstone is mounted in the hands or face of a watch or other time-piece and is therefore covered by the watch- glass. This can be a problem where the watch-glass comprises a material which also luminesces under UV light.

Synthetic colourless sapphire for instance, is sometimes used as a watch-glass material mainly because of its high scratch resistance and good optical transmission in the visible region of the spectrum. Watch glass grade sapphire has sufficiently low absorption coefficient at wavelengths between 216 and 225 nm to allow diamond to be excited through sapphire watch glass at energies above its bandgap energy.

Synthetic colourless sapphire is known to luminesce at wavelengths of 426 nm, due to the presence of titanium 4+ complexes, and/or at 692 nm, due to the presence of chromium 3+ complexes. Figures 1 a to 1 c illustrate luminescence emitted by five different samples of synthetic colourless sapphire watch-glass under or following UV excitation.

Figure 1 a illustrates fluorescence emitted during excitation by a UV excitation pulse. As can be seen from the Figure, fluorescence emitted by all five samples of watch-glass is negligible. Figure 1 b illustrates phosphorescence emitted by the five samples of watch-glass in a first time window of between 90 με and 40 ms after the end of the UV excitation pulse. Figure 1 c illustrates phosphorescence emitted by the five samples of watch-glass in a second time window of between 70 ms and 140 ms after the end of the UV excitation pulse. It will be appreciated that as discussed above, these images may be produced by combining multiple image captures. All five samples exhibit blue phosphorescence in at least the first time window.

As illustrated in Figures 1 a to 1 c, the 426 nm blue phosphorescence from the sapphire watch-glass can occur across a wide range of timescales (sub-microsecond to seconds), depending on the complexes present in the sapphire. This emission can therefore potentially obscure the blue fast phosphorescence marker described above, preventing accurate identification by known means of one or more diamonds mounted beneath the watch-glass. In such cases, the diamond under test would need to be referred to an alternative apparatus for further testing. This further testing may not be possible without removing the diamond from its mounting.

Summary In accordance with one aspect of the present invention there is provided a method of identifying a mounted diamond as natural or synthetic. The method comprises irradiating the diamond with at least one excitation pulse of ultra-violet radiation. During and/or following the excitation pulse, light emitted by the diamond in at least one time window having a predetermined time relationship relative to the excitation pulse is detected so as to obtain luminescence data. The or each time window is chosen to include luminescence having a decay time characteristic of one or more one or more specific markers in the luminescence properties of the diamond. The method further comprises processing the luminescence data to remove light emitted at wavelengths shorter than a specified wavelength. Further aspects and preferred features are set out in claim 2 et seq.

Brief Description of the Drawings Figures 1 a to 1 c illustrates blue phosphorescence emitted by five different samples of synthetic sapphire watch-glass;

Figure 2 illustrates an apparatus for identifying mounted gemstones;

Figure 3 illustrates fluorescence emitted by three gemstone samples;

Figures 4a to 4d illustrate phosphorescence emitted by a sample of sapphire watch- glass and three gemstone samples; and

Figures 5a and 5b illustrate further phosphorescence emitted by a sample of sapphire watch-glass and three gemstone samples.

Detailed Description

A method of and an apparatus for identifying mounted gemstones, such as diamonds, as natural or synthetic, will now be described with reference to Figures 2 to 5. The diamonds to be identified may be mounted in an article such as a watch. The diamonds will typically comprise polished stones, both brilliant and fancy cut.

The method may be carried out using an apparatus substantially as described in PCT/GB2016/051927. The apparatus 300 is illustrated in Figure 2 and comprises a source of electromagnetic radiation 130 (such as a microsecond xenon spark lamp) at wavelengths of substantially 225 nm or less, a light detection device 140 (such as a charge coupled device (CCD) camera) for capturing any visible light emitted by the diamond 160 and a control system (not shown) to synchronise the operation of the source 130 and light detection device 140. The control system configures the source 130 to repeatedly irradiate the diamond 160 with multiple excitation pulses of electromagnetic radiation, and the light detection device 140 to capture any visible light emitted to produce image data during time windows, each having a predetermined time relationship relative to an excitation pulse, each time window being closed before the start of the next excitation pulse, so as to obtain luminescence data. A processor (not shown) may be configured to combine the luminescence data from the multiple associated with all of the pulses.

The diamond sample 160 to be tested is held on a sample holder 170 within a chamber 150. The surface of the sample 160 is irradiated by the electromagnetic source (lamp) 130. The source 130 and the light detection device 140 are configured by the control system to repeatedly irradiate the diamond 160 and capture any visible light emitted in multiple exposure windows to produce multiple captures. The light detection device 140 can be configured by the control system with a delay, such that the camera captures visible light during a time window which opens after the associated excitation pulse has ended. Thus, any fluorescence emitted by the diamond sample 160 is filtered out. Such fluorescence would potentially mask any short-lived phosphorescence. The length of the delay in recording (i.e. the time window start time relative to the excitation pulse start) can be set by an operator, as can the length of recording (i.e. length of time window) and the number and/or frequency of excitation pulses, using the control system. One or more of these parameters may be operator controllable via the control system. As previously discussed, synthetic sapphire watch-glass may exhibit blue phosphorescence following UV excitation, and these emissions may obscure the blue fast phosphorescence marker, with a decay of less than about 80 ms. However, where the watch-glass exhibits no fluorescence during excitation, known fluorescence markers may still be used for identification of the diamond where appropriate.

Figure 3, which shows fluorescence emitted during an excitation pulse, illustrates how an HPHT synthetic diamond can be identified by a very weak green fluorescence marker, as indicated by the dotted circle. The weak green fluorescence marker has a wavelength of about 510nm, and testing for the weak green fluorescence marker may comprise testing in a time window synchronised (i.e. coincident) with an excitation pulse. The end of the time window will be carefully selected to coincide with the end of the pulse, so that any phosphorescence emitted after excitation of the diamond has terminated, which might otherwise obscure the detection of this weak fluorescence, will be "gated out". However, for other diamond types different markers must be used for identification. It will be appreciated from Figures 1 a to 1 c that the blue phosphorescence emitted by synthetic sapphire watch-glass is substantially uniform across the sample. It is therefore possible to identify this phosphorescence and "substract" the light at that wavelength from an image of the sample, taken using the apparatus 300 described above.

Figures 4a shows a sample of sapphire watch-glass and three diamond samples exhibiting phosphorescence in a first time window of 90 με to 40 ms following the end of an excitation pulse. Figure 4b shows the same samples of watch-glass and diamond exhibiting phosphorescence in a second time window of 70 ms and 140 ms following the end of the excitation pulse. Figure 4c shows the same samples of watch glass and diamond during the same, first time window, but with the blue background "subtracted". Similarly, Figure 4d shows the same sample during the same, second time window, but with the blue background "subtracted". The blue back ground may be "subtracted" using appropriate software, as is known in the art.

As illustrated in Figures 4c and 4d, a small level of blue phosphorescence from the natural diamond is seen. This may be due to reflections or to the broad 426 nm light from the watch glass exciting defects in the diamond. It should also be noted that, after the watch-glass phosphorescence has been subtracted (as in figures 4c and 4d), a stone may appear to show blue phosphorescence either because it reflects back the phosphorescence from the watch glass or because it is actually emitting the phosphorescence itself (or a combination of the two).

A further marker that may be used in the identification of a mounted diamond is a green slow phosphorescence marker. This marker comprises phosphorescence having a wavelength between about 530 nm and about 550 nm, and a decay time greater than 80 milliseconds after the end of an excitation pulse. Testing for this marker may involve testing, in a time window opening about after the end of the associated excitation pulse, for a luminescence band between about 530 nm and about 550 nm. The time window may optionally close about 500 ms after the end of the associated excitation pulse. The presence of the green slow phosphorescence marker may be an indicator that the diamond is a synthetic HPHT or CVD diamond. Figures 5a and 5b show the sample of sapphire watch-glass and three diamond samples of Figures 4a to 4d in the first (90 με to 40 ms after excitation) and second (70 ms and 140 ms after excitation) time windows respectively. The green channel of the light detection device or camera 140 has been isolated from the red and blue channels such that only green emissions are shown. It will be understood that this process removes from the images the blue phosphorescence emitted by the sapphire watch- glass, enabling the green slow phosphorescence marker described above to be identified. As can be seen in Figures 5a and 5b, for the natural diamond, a small amount of light will hit the green channel in the camera 140 due to a small overlap from the 450 nm emission and green channel response. However, only the synthetic diamonds will show a significant green component. Similarly, the red channel may be isolated from the camera's blue and green channels so that a red phosphorescence marker is visible. The red phosphorescence marker comprises luminescence having a wavelength between about 575 nm and about 690 nm, and a decay time greater than 1 millisecond. The presence of the red phosphorescence marker may be an indicator that the diamond may be a synthetic HPHT or CVD diamond.

Alternatively, the blue channel may be isolated from the red and green channels and rejected, such that only red and green emissions appear in an image for analysis. As described above, any green or red phosphorescence having a decay time of greater than 80 ms or 1 ms respectively after the end of an excitation pulse may indicate that the diamond under test is a synthetic diamond.

The method of identification of mounted diamonds described herein therefore utilises one or more of fluorescence data and processed phosphorescence data, in order to identify known markers in the luminescence properties of a diamond. It will be appreciated that these techniques may be used in combination in order to accurately identify a mounted diamond as natural or synthetic. It will be appreciated by the person skilled in the art that various modifications may be made to the above described embodiment, without departing from the scope of the present invention. As used herein, natural is defined as a stone from nature consisting exclusively of diamond produced by geological processes. The term natural, as defined herein, indicates that the stone is not synthetic, but does not exclude the possibility that the stone could have been treated, for example by pressure or heat treatment, unless specifically stated.

As used herein, synthetic is defined as a man-made stone consisting exclusively of diamond produced by artificial or industrial processes, such as chemical vapour deposition or high pressure high temperature processes. As used herein, treated is defined as a natural stone (as defined above) which has been modified in order to improve its colour or clarity, for example by chemical or mechanical means, by irradiation or by pressure or heat treatments.

As used herein, type is defined using the standard diamond classification system which separates stones based on their physical and chemical properties, e.g. Type la, Type lib etc.

As used herein, mounted is defined as set into jewellery, such as a ring, necklace, bracelet, brooch, earring and the like, or set into an object such as a watch, a phone case, a frame, a box or the like.

Claims

CLAIMS:

1. A method of identifying a mounted diamond as natural or synthetic, the method comprising:

irradiating the diamond with at least one excitation pulse of ultra-violet radiation; during and/or following the excitation pulse, detecting light emitted by the diamond in at least one time window having a predetermined time relationship relative to the excitation pulse so as to obtain luminescence data, the or each time window being chosen to include luminescence having a decay time characteristic of one or more one or more specific markers in the luminescence properties of the diamond; and processing the luminescence data to remove light emitted at wavelengths shorter than a specified wavelength.

2. A method as claimed in claim 1 , wherein the diamond is mounted beneath the watch-glass of a time-piece.

3. A method as claimed in claim 2, wherein the watch-glass comprises synthetic sapphire that, when irradiated with ultra-violet radiation, emits light at of around 426 nm.

4. A method as claimed in any one of the preceding claims, comprising detecting light emitted by the diamond following the excitation pulse, wherein the specified wavelength is around 435 nm.

5. A method as claimed in any one of the preceding claims, comprising detecting light emitted by the diamond following the excitation pulse, wherein the specified wavelength is around 510 nm.

6. A method as claimed in any one of the preceding claims, comprising detecting light emitted by the diamond following the excitation pulse, wherein the specified wavelength is around 510 nm and the processing further comprises removing light emitted at wavelengths longer than 570 nm.

7. A method as claimed in any one of the preceding claims, comprising detecting light emitted by the diamond following the excitation pulse, wherein the specified wavelength is around 570 nm and the processing further comprises removing light emitted at wavelengths longer than 700 nm.

8. A method as claimed in any one of the preceding claims, wherein one of the one or more markers is a blue fast phosphorescence marker comprising luminescence in a wavelength band peaking at about 450 nm and a decay time of less than about 80 ms, and wherein testing for the blue fast phosphorescence marker comprises testing, in a time window opening at or after the end of the excitation pulse and ending about 80 milliseconds after the end of the associated excitation pulse, for a luminescence band peaking at about 450 nm.

9. A method as claimed in any one of the preceding claims, wherein one of the one or more markers is a green slow phosphorescence marker comprising luminescence having a wavelength between about 530 nm and about 550 nm and a decay time greater than 80 milliseconds, and wherein testing for the green slow phosphorescence marker comprises testing, in a time window opening about after the end of the associated excitation pulse, for a luminescence band between about 530 nm and about 550 nm, the time window optionally closing about 500 ms after the end of the associated excitation pulse.

10. A method as claimed in any one of the preceding claims, wherein one of the one or more markers is a red phosphorescence marker comprising luminescence having a wavelength between about 575 nm and about 690 nm and a decay time greater than 1 millisecond, and wherein testing for the red phosphorescence marker comprises testing, in a time window opening at the end of the associated excitation pulse, for a luminescence band between about 575 nm and about 690 nm, the time window optionally closing about 500 ms after the end of the associated excitation pulse.