GB2286760A - Colouration of gemstones - Google Patents

Abstract

Gemstones, e.g. topaz, having more than one polarisation sensitive absorption peak affecting light in the visible region of the spectrum are cut and mounted so as selectively to enhance the effect of one absorption peak relative to the other or another. The effect is provided, or further enhanced by a polarisation filter 12, which may be adjustable by a wearer, incorporated in the mount 13 for the stone 11. A cylindrical light shield 15 is positioned to limit incidence of light from the side. Ionising radiation is used to create or enhance the polarisation sensitive absorption peaks in the gemstone. <IMAGE>

Description

Colouration of Gemstones The invention relates to a method of producing colouration effects in a gemstone, and in topaz in particular, and to a product gemstone produced by the method. More particularly the invention relates to a method for facilitating control of colouration effects produced by exposing a gemstone to ionising radiation (which term as used herein is to be understood as including neutron radiation) and to mounting and cutting arrangements for controlling the observed effects of the colouration. The latter also have application for gemstones having naturally occurring polarisation sensitive colourations.

Effects of bombardment of various gems, glasses and plastics by sub-atomic particles are discussed in EP 0 249 038. Prior to that reference is made in an article by K. Nassau in Gems and Gemology, Spring 1985, to alterations in the colour of topaz by exposure to X-rays, gamma rays, neutrons and high energy charged particles such as electrons, protons and the like. The effects of gamma, neutron and high energy electron irradiation are discussed in some detail. Irradiation with ions was not considered and has not been regarded as a practical option since it suffers from all the drawbacks (generation of heat, limited penetration or excessive surface damage) of electron irradiation, but to a much greater degree.

The physical mechanisms involved in colouration in gemstones both naturally and when produced by ionising radiation are not comprehensively understood. Mechanisms are proposed in an article by K. Schmetzer entitled "Irradiation-induced Blue Colour in Topaz" in Naturwissenschaften 74 (1987) and an article by Vann Priest et al entitled "A Dangling-Silicon-Bond Defect in Topaz" in the Journal of Applied Physics 68(6) 15th September 1990. These articles also disclose a polarisation dependency of absorption bands found in irradiated topaz.

There remain, however, a number of unpredictable effects obtained when topaz is subjected to ionising irradiation. For example, it is evident that the colouration produced by exposure to gamma rays or high energy electrons is dependent upon the natural properties of the stones. Exposure to gamma rays can produce colours which appear yellow, reddish-brown or brown at doses less than 1 megarad. At higher doses some stones will develop a green-yellow to green-blue colour, and a blue colour may be brought out in such stones by heat treatment.

Where the blue colour is particularly intense, that produced by gamma irradiation tends to be grey or steelblue, whilst high energy electron irradiation of such stones produces a clear sky-blue.

Neutron irradiation, on the other hand, can be relied upon to turn pale or colourless topaz blue if a sufficient dose is given, but the blue may have a steely or grey character.

Now we have found that electron irradiated topaz exhibits three absorption bands in the visible region of the spectrum when viewed with plane polarised light. One absorption band peaks at 625 nanometres when the plane of polarisation is parallel to the b axis. The other two absorption bands both peak at 600 nanometres when viewed with light polarised parallel respectively to the a and c axes. The relative intensities for the three absorption bands for light polarised parallel to albic are respectively 0.2/1.0/0.3. When viewed with unpolarised light these absorption bands g ve rise to a light-blue colour along the b axis and a darker colour when viewed in the direction of the other two axes.

Furthermore, since the peak position with the light polarised parallel to the b direction is displaced towards the red end of the spectrum compared with those observed with the light polarised parallel to the other two directions, the resultant colour for light polarised in the b direction is turquoise (i.e.blue-green) compared with blue for the light polarised in the other two directions.

Using unpolarised light, both the effect of variation of depth of colour with viewing direction and the pleochroism are preserved, though to a somewhat lesser extent. The colour intensity is approximately 2.5 times greater when viewed in the a and c directions than when viewed in the b direction and is noticeably more turquoise in the a and c directions.

On the basis of these observations, we have derived that the blue colour intensity of a gemstone treated in this way can be emphasised by cutting the gemstone so as to maximise viewing of light entering and leaving in the crystalline b direction. Correspondingly the turquoise hue can be emphasised by maximising the viewing of light polarised parallel to the crystalline a or c direction.

By cutting facets at prescribed angles, it is possible to suppress reflection of light polarised in directions other than that desired. Correspondingly, preferred reflections may be enhanced by applying reflecting coatings to selected facets of the cut gemstone.

Alternatively or additionally a polarising filter may be incorporated in a mount for the gemstone, positioned so as to improve a desired hue by optimising the polarisation of light transmitted or reflected through the gemstone in the principal viewing direction or directions.

Further, we have now found that ion bombardment of topaz produces absorption bands with peaks at the same positions as electron bombardment. The peak height of the band at 625 nanometres is, however, increased relative to the peak height of the 600 nanometre bands.

As a result, the orientation effects of colour intensity and the pleochroism observed are enhanced as compared with these effects produced in electron irradiated material.

The article referred to above by Vann Priest et al reports production by gamma ray irradiation of absorption bands with peaks at 620 nanometres for light polarised in all three directions but with varying peak heights. From this, we derive that a strong colour will be observed in the a and c directions whilst the colouration would be weak in the b direction with unpolarised light. However, the hue would not vary with orientation.

Our observations have shown that neutron irradiation, on the other hand, produces a strong absorption band with light polarised parallel to the b direction at 630 nanometres together with weaker absorption bands having peaks at 605 nanometres for light polarised in the a or c direction. There results a marked effect of orientation upon both intensity and colour. Given the current experimental error in these measurements it is possible that these bands induced by neutron irradiation correspond with those produced by electron and ion irradiation, as noted above.

In addition, we have now found that strong ultraviolet absorption bands are observed in natural topaz. These absorption bands are further enhanced on irradiation with electrons and ions. We would expect there to be a similar enhancement on irradiation with gamma rays or neutrons. Where such a band or bands is/are particularly strong, and the long-wavelength tails extend into the visible region they will influence the colour.

Thus, if the tail of the ultraviolet absorption band extends sufficiently into the blue region of the spectrum it reduces the blue transmitted light with respect to the green and will result in a colouration which appears more green.

According to one aspect of the invention these characterisations are applied in selective irradiation using one or a selected combination from neutrons, ions, gamma rays and electrons and combining this with selective cutting and/or mounting to tailor the colour hue and intensity seen in the product gemstone.

Accordingly the invention provides, in one of its aspects, a gemstone having more than one polarisation sensitive absorption peak affecting light in the visible region of the spectrum at least one of which peaks has been produced or enhanced by exposure to ionising radiation wherein provision is made in a mounting for the gemstone, and/or by choice of angle of cut of at least one facet relative to the crystallographic axes of the gemstone, so that there is a selective enhancement of the effect upon visible light emerging from the gemstone of one said absorption peak relative to the other or another of said absorption peaks.

The mounting may be provided with m.eans for polarising light, such as a polarising filter positioned between the gemstone and a component of the mounting, so as to polarise light which passes into the gemstone by transmission through or reflection from the mounting, the polarisation being so chosen in relation to the orientation of the gemstone as to effect the said selective enhancement. If, for example the polarisation is parallel with the b crystallographic axis in a topaz stone, the effect of the absorption which peaks at 625 nanometres will be emphasised. If, on the other hand, the polarisation is parallel with the a or c crystallographic axis, the effect of the absorption which peaks at 600 nanometres will be emphasised.

It is a further aspect of the invention that the use of such a means for polarising light has application in the mounting of gemstones having more than one naturally occurring polarisation sensitive absorption peak.

Preferably, the said mounting is provided with means for adjusting the plane of polarisation relative to the orientation of the gemstone in order that the said selective enhancement effect can be transferred from one said absorption peak to another.

The invention provides, in another of its aspects, a method of producing a colouration in a gemstone of a type in which more than one polarisation sensitive absorption peak affecting light in the visible region of the spectrum is produced or enhanced by exposure to ionising radiation, which method comprises exposing the gemstone to ionising radiation so as to create or enhance said absorption peaks and providing a mounting, and/or choosing the angle of cut of at least one facet relative to the crystallographic axes of the gemstone so that there is a selective enhancement of the effect upon visible light emerging from the gemstone of one said absorption peak relative to the other or another of said absorption peaks.

It will be appreciated that the choice of the angle of cut of the said facet is made from knowledge of the relationships described above between the crystallographic axes and the respective planes of polarisation to which the absorption peaks are sensitive.

It is thus possible for the facets to be cut either before or after the exposure to ionising radiation.

There is a large number of inter-linked parameters, including energy, mass (where appropriate), dose, dose rate and treatment temperature, from which a selection has to be made in specifying the exposure to ionising radiation which is to produce a particular desired result. Thus, the depth of penetration is dependent upon the choice of irradiation (gamma, electron, neutron or ion), increasing with increasing energy of the irradiation but, in the case of ions, decreasing with increasing ion mass. The resultant colouration intensity will be greater, the greater the total energy deposition, which can be increased by increasing the dose. On the other hand, a lower dose of heavier ions may produce an equivalent intensity of colouration to that produced by a higher dose of more penetrating radiation such as gamma or electron radiation.Again, if the gemstone is small, there is obviously a limit to the depth of penetration that it is sensible to aim for.

A lower limit of energy and dose is set in that there must be a sufficient combination of penetration and colouration to generate a worthwhile visible effect, which we define as that which produces an absorption coefficient of 5 x 10-3cm-1.

Upper limits of energy may be set by the level at which unacceptable damage, or activation of the host gemstone material occurs. Or the energy level may be limited by that which is available from conventional sources. In general, a higher energy input will enable a given colour be achieved with a lower dose.

Upper limits of dose are generally set by effects produced by the irradiation reaching a saturation.

All of these considerations are affected by the size of stone being treated and the nature of the stone itself. Assuming the energy of the electrons is sufficient to penetrate completely the stone being treated, a dose of electrons as low as 103 Megarads can produce a pale colouration in a 5mm cube of topaz. A similar colouration would be produced in a larger stone by a lower dose. It is not therefore realistic to define a minimum practicable dose, although it is unlikely that a large enough topaz stone would be treated and show an acceptable colouration at a dose less than 10 Megarads.

For practical reasons such as saturation effects it is unlikely that a requirement for a dose in excess of 5 x 105 Megarads would be encountered.

Somewhat similar considerations apply to gamma irradiation in that 10 Megarads represents the lowest dose that might reasonably be expected to generate an acceptable effect in a large stone. A dose of 5 x 105 Megarads would be expected to produce saturation in any practical application, although, using gamma radiation produced by a typical Co60 source (e.g. 1 Megarad per hour) the dose would have to be two orders of magnitude smaller to represent a realistic target.

For neutrons, the energy spectrum is determined by the source, typically a nuclear reactor. Exposure to 3 x 1016 neutrons cam~2 in an appropriate energy range will typically create a pale colour in a 5mm cube of topaz whereas exposure to 3 x 1017 n.cm~2 will generate a deep colour.

Following the same considerations given above for electron irradiation, exposure to less than 3 x 1014n.cm-2 or more than 1019 n.cm~2 is unlikely. It has also to be remembered that neutron irradiation may activate impurity elements in the topaz which will have to be stored under appropriate shielding for several months until the activity has decayed to safe levels.

When irradiating with ions, a further variable is introduced in that different ions of different mass can be used for the bombardment. An upper limit for the energy of ion irradiation is set by the level at which the bombardment activates the host gemstone material.

This upper limit varies from a few tens of MeV per nucleon for light ions to a few MeV per nucleon for heavy ions as illustrated by the following table for topaz irradiation: Ion Upper Energy Limit (MeV) 1H 40 19F 80 35C1 100 56Fe 150 Dose to saturation also varies with ion mass as indicated by the following table: Ion Saturation Dose (ions,cm-2) 1H 6 x 1018 19F 2.5 x '?15 35C1 7.5 x 1013 56Fe 4 x 1013 Specific examples of methods and products embodying the invention will now be described Samples of colourless topaz were subjected to irradiation with protons produced by an ion accelerator.

The ion energy was 10 MeV and the dose 5 x 1016 ions cam~2.

For comparison a corresponding selection of samples of colourless topaz were subjected to irradiation with electrons at an energy of 20 MeV and to a dose of 104 Megarads.

This revealed that the proton irradiation produced an optical absorption coefficient on average three times that produced by the electron treatment.

The two absorption bands which peak at 600 nanometres and 625 nanometres respectively are produced by both the electron and the ion irradiation. Not only is the optical absorption coefficient greater following the ion irradiation as compared with electron irradiation but also the ratio of the absorbance values of the two bands is different depending upon whether ion or electron irradiation has been used. Consequently the resulting colour is potentially different, depending upon the orientation of the topaz. In the optimum viewing orientation blue is more prevalent in the topaz treated with electrons and turquoise is more prevalent in the topaz treated with protons.These effects can be modified or enhanced by cutting and mounting arrangements as described below with reference to Figure 1, which shows in cross section a cabachon of topaz and illustrates the relationship between the cut surface, the crystallographic axes, and a mounting which includes a polarising filter.

The intensity of colour observed in the product is, of course, dependent not only upon the optical absorption coefficient but also upon the depth of penetration, which for a given energy is greater for electrons than for protons. In general the penetration at a given energy is related to the mass of the particle, being less the heavier the ion. To some extent this effect can be offset by increasing the energy for heavier ions, but as discussed above a limit is set when the extent of the damage (e.g. to the crystal structure) which follows produces unacceptable effects in the product.

Thus, ion treatment, or a combination of ion and electron treatment could be particularly advantageous for small stones in which exposure to electrons alone fails to produce an acceptable colour.

Figure 1 illustrates a topaz stone 11 which has been cut and mounted to enable a selected colour hue to be emphasised. This may be a stone which has been exposed to electron irradiation, but if it is desired to produce a colour shifted further towards green, then we would expose the stone to proton irradiation.

Referring to the drawing, a circular polarising filter 12 is positioned between cabachon 11 and a support 13 which has a reflecting surface 14. The mounting is provided with a cylindrical light shield, shown diagrammatically at 15, positioned to limit incidence of light from the side.

The arrow a indicates the axis of symmetry of the cabachon extending at right angles to the flat base which rests on the polarising filter 12. The topaz is cut so that the crystallographic axis a coincides with this axis marked by the arrow a. The b and c crystallographic axes lie in the plane of the base.

With this configuration unpolarised white light entering the gemstone in the a direction is subject to strongest absorption. Reflected light is plane polarised by the filter 12. Rotation of filter 12 will alter the relative absorption of the total incident light by the respective absorption centres at 625 nm and 600, thus altering the hue of the stone seen from above.

The polarising filter 12 may be rotated during manufacture to set a desired hue and then fixed.

Alternatively the polarising filter 12 may be mounted so as to be rotatable, thus allowing the wearer to select the hue at will by appropriate rotation of the filter.

If it is desired to produce a mounted topaz gemstone with a fixed turquoise hue shifted as strongly as practicable towards green, this, we have found, can be achieved by careful annealing. Thus, the 600 nm absorption peaks anneal more readily at 2500C than the 625 nm absorption peak. The 600 nm absorption peaks absorbance values decreased by 43% after 1 hour at 2500C whereas the 625 nm absorption peak absorbance value decreased by only 15%. The ratio of absorbance values is thus altered in favour of producing a more green colour when viewed in white light.

The invention is not restricted to the details of the foregoing examples.

Claims (18)

Claims

1. A gemstone having more than one polarisation sensitive absorption peak affecting light in the visible region of the spectrum at least one of which peaks has been produced or enhanced by exposure to ionising radiation wherein provision is made in a mounting for the gemstone, and/or by choice of angle of cut of at least one facet relative to the crystallographic axes of the gemstone, so that there is a selective enhancement of the effect upon visible light emerging from the gemstone of one said absorption peak relative to the other or another of said absorption peaks.

2. A gemstone as claimed in Claim 1 wherein the said mounting is provided with means for polarising light which passes into the gemstone by transmission through or reflection from the mounting, the plane of polarisation being so chosen in relation to the orientation of the gemstone as to effect the said selective enhancement.

3. A gemstone as claimed in Claim 2 wherein the said means for polarising light comprises a polarising filter positioned between the gemstone and a component of the mounting through which or from which light is respectively transmitted or reflected into the gemstone.

4. A gemstone as claimed in Claim 2 or Claim 3 wherein the gemstone is topaz.

5. A gemstone as claimed in Claim 4 wherein the said means for polarising light is so arranged that the light passing through it into the gemstone is polarised parallel with the b crystallographic axis so as to emphasise the effect of the absorption which peaks at 625 nanometres on the colour observed in the gemstone.

6. A gemstone as claimed in Claim 4 wherein the said means for polarising light is so arranged that the light passing through it into the gemstone is polarlsed parallel with the a or c crystallographic axis so as to emphasise the effect of the absorption which peaks at 600 nanometres on the colour observed in the gemstone.

7. A gemstone as claimed in any of Claims 2 to 6 wherein the mounting is provided with means for adjusting the plane of polarisation relative to the orientation of the gemstone in order that the said selective enhancement effect can be transferred from one said absorption peak to another.

8. A method of producing a coloration in a gemstone of a type in which more than one polarisation sensitive absorption peak affecting light in the visible region of the spectrum is produced or enhanced by exposure to ionising radiation, which method comprises exposing the gemstone to ionising radiation so as to create or enhance said absorption peaks and providing a mounting, and/or choosing the angle of cut of at least one facet relative to the crystallographic axes of the gemstone, so that there is a selective enhancement of the effect upon visible light emerging from the gemstone of one said absorption peak relative to the other or another of said absorption peaks.

9. A method as claimed in of Claim 8 in which the said ionising radiation comprises electrons having an energy and a dose sufficient to provide an absorption peak with an absorption coefficient of at least 5 x 10-3cm-1.

10. A method as claimed in Claim 8 in which the said ionising radiation comprises ions, the mass, and dose of which are chosen so as to provide an absorption peak with an absorption coefficient of at least 5 x 10~3cam~1.

11. A method as claimed in Claim 10 in which the ionising radiation comprises protons.

12. A method as claimed in Claim 10, in which the ionising radiation comprises neutrons of appropriate energy and to a dose sufficient to provide an absorption peak with an absorption coefficient of at least 5 x 103cm-1.

13. A gemstone when made by the method of any one of Claims 8 to 12.

14. A gemstone having more than one polarisation sensitive absorption peak affecting light in the visible region of the spectrum supported on a mounting which incorporates means for polarising light which passes into the gemstone by transmission through or reflection from the mounting.

15. A gemstone as claimed in Claim 14 in which the said means for polarising light comprises a polarising filter the position of which is adjustable by a wearer of the mounted gemstone.

16. A gemstone as claimed in Claim 14 in which the said means for polarising light comprise a polarising filter the position of which is fixed during mounting of the gemstone so that there is a selective enhancement of the effect upon visible light emerging from the gemstone of one said absorption peak relative to the other or another of said absorption peaks.

17. A gemstone substantially as hereinbefore described with reference to, and illustrated in, the accompanying drawing.

18. A method substantially as hereinbefore described.