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by gagan choudhary
A suite of synthetic sapphires with ‘natural-like’ sheen
Author: Gagan Choudhary
(This article was first appeared in Gems & Jewellery, Vol. 18, No. 3, pp 6-9)
‘Sheen’ is commonly known as a moving white light or milkyness on the surface caused due to reflection and scattering of light from minute inclusions. Such effect is a common feature in feldspars (especially moonstone), however can be observed in any gem provided such inclusions are present; other common gem material is corundum where ‘sheen’ is caused as a result of presence of minute inclusions of titanium oxide in the form of fine silk, discs or dust. Presence of such inclusions is usually indication of a stone being natural. However, since the production of synthetic star sapphires and rubies, rutile inclusions are being produced in synthetic corundum by inducing or diffusing titanium oxide in the lattice. Such inclusions when properly oriented and present in higher concentrations give rise to a star effect (when cut as a cabochon) and if present in lower concentrations produce only milkyness.
In January 2009, at the Gem Testing Laboratory of Jaipur, we were able to procure few specimens of synthetic sapphires displaying sheen effect for study and reference purpose. The procured set consisted four specimens, two yellow, one green and one greenish blue; all were faceted as octagon step and ranged in weight from 2.17 to 3.41 carats (figure 1.a). In addition, a 2.14 carats synthetic yellow sapphire (figure 1.b) was submitted to the laboratory for identification.
Results and Discussion
Visual Characteristics
The procured samples were yellow, green and greenish blue having a good transparency. All four samples displayed sheen like effect on the table facet (again, figure 1.a); no other part of the stones displayed this effect. As a result all the samples appeared slight hazy from the table. The greenish blue sample gave an ‘electric’ look similar to that seen in ‘Paraiba’ type tourmalines. The green and greenish blue sample also displayed eye-visible blue coloured curved zones of pinpoints which immediately identified the stones as product of flame fusion or vernueil process. The blue zones in one of the samples (figure 1.a) were quite broad and dense and gave appearance of a ‘parti-coloured’ stone with blue and green colours; yellow samples were uniformly coloured.
Figure 1.a: These synthetic sapphires displaying natural like sheen effect was procured for study and reference purposes. Note the sheen effect restricted on the table, which gives rise to a suspicion regarding the origin. Also note the curved zones in the greenish blue sample.
Figure 1.b: This synthetic sapphire submitted for identification at the Gem Testing Laboratory, Jaipur contained fine milky zones as commonly observed in sapphires of natural origin
Gemmological Properties
The gemmological properties of the studied set of synthetic sapphires are described below and are listed in table 1. RI and SG values obtained were consistent with those of corundum - natural or synthetic.
All specimens displayed anisotropic nature with strong interference colours visible at the table facet. Using a conoscope, ‘Uniaxial’ optic figure was readily resolved. This indicated the ‘c’ axis is oriented perpendicular to the table facet. All specimens displayed a chalky blue fluorescence under shortwave ultraviolet while inert under longwave. This chalky fluorescence was restricted only to the table facet (figure 2) as a result of concentration of fine inclusions giving rise to milkyness or sheen effect. Under chelsea filter, the greenish blue sample showed a strong red reaction with stronger intensity towards the blue bands, while green sample displayed a weak red only towards the corners and yellows did not exhibit any changes. Under desk-model spectroscope, the greenish blue variety displayed a typical cobalt spectrum with three bands at around 540, 580 and 630 nm, while green sample displayed only fine lines at the red end. No absorption features were seen in yellow samples.
Figure 2: The ultraviolet fluorescence of samples in figure 1.a under shortwave revealed strong chalky blue glow which was restricted at the sheen producing inclusions near the table facets.
Shortwave ultraviolet transparency
This test is quite useful for separating light coloured natural and synthetic corundum where synthetics are transparent while naturals are opaque under shortwave ultraviolet. However, presence of impurities in a synthetic sapphire will make it appear opaque as was in this case. The opacity in these sapphires was due to the presence of fine whitish inclusions giving milkyness. The specimen in figure 1.b, which lacks such impurities appeared transparent under shortwave ultraviolet (figure 3).Figure 3: The sample in figure 1.b, which was submitted for identification and displayed milky zones was confirmed using shortwave ultraviolet transparency test. Note the transparent centre and dark edges
Table 1: Gemmological properties of three different colours of synthetic sapphires displaying natural like ‘sheen’.
Microscopic features
In all samples, fine minute inclusions of variable pattern were present restricted to the table facet. These mainly consisted of oriented short needle like (figure 4, left) as well as fine pinpoints similar to dust (figure 4, right); both these types gave rise to the sheen effect. These pinpoint (dust) or needle like inclusions similar in appearance to those observed in star corundum, either synthetic or natural with induced inclusions. Such inclusions are formed as result of exsolution of titanium oxide in the structure of corundum. Therefore, these inclusions were presumed to be of rutile, which was well supported by the EDXRF analyses.
Figure 4: Fine minute inclusions of variable pattern were present restricted to the table facets consisting of short needles (figure 4.left) as well as fine pinpoints similar to dust (figure 4.right); both these types gave rise to the sheen effect. Such inclusions are commonly observed in synthetic star corundum or natural corundum with induced or diffused titanium oxide.
In green and greenish blue samples, thick and distinct curved bands were visible even with unaided eyes (see again, figure 1.a). Although, these bands appeared blue but these were quite different from those usually seen in synthetic blue sapphires. These were composed of blue coloured droplets (figure 5) which were very fine as well as coarse in appearance. EDXRF analyses of these samples revealed a high concentration of cobalt, therefore, the possibility of these blue droplets corresponding to cobalt impurity cannot be ruled out.
Figure 5: The blue zone in the greenish blue sample consisted of fine to coarse blue coloured droplets. These gave rise to the bluer body colour on the original green stone.
In addition to the pinpoints described above, these synthetic sapphires also exhibited fine whitish pinpoints similar to gas bubbles or unmelted feed powder which was arranged in curved zones or clouds. These zones were mainly oriented along the axis running parallel to the table facets (figure 6). Such curved zones are formed along the length of the boule and represents the growth direction. However, in these samples, the ‘optic’ axis or the ‘c’ axis and the growth direction of the boule were at 90o to each other. The optic axis or the ‘c’ axis was oriented perpendicular to the table as ‘uniaxial’ optic figure was seen at the table, while the axis with circular zones parallel.
Figure 6: The axis of curved zones of dotted inclusions was oriented parallel to the table facets. This indicates that that the growth direction of the crystal (boule) and ‘c’ axis is perpendicular to each other.
The specimen in figure 1.b, which was submitted for identification displayed fine and sharp milky zones (figure 7) composed of fine pinpoints, however not readily resolved. Such zones are typically encountered in sapphires of natural origin and could have easily misidentified the stone as natural. In addition to the minute gas bubbles presented in zones, some scattered spherical gas bubbles were also encountered. Further to the typical features associated with synthetic corundum, few uncommon inclusions were also observed. These comprised some wavy clouds (figure 8) composed of fine dotted inclusions and appeared white. Such clouds however did not follow any crystallographic orientations and appeared as if some stress cracks have been filled up by these dotted inclusions / pinpoints.
Figure 7: The specimen in figure 1.b, displayed fine milky straight zones as commonly observed in natural sapphires. Absence of other features or a careless testing may have been misidentified the stone as natural. Its synthetic origin was determined by using SWUV transparency test.
Figure 8: Some randomly oriented wavy clouds composed of whitish dotted inclusions were also present in the greenish blue sample.
In the green sample, one surface-reaching, fingerprint-like inclusion was observed (figure 9). It had a whitish appearance, suggesting they contained a foreign substance, as commonly seen in corundum exposed to high-temperature heating. In addition it also appeared iridescent as in case of natural sapphires.
Figure 9: In the green sample, one surface-reaching, fingerprint-like inclusion was observed with a whitish appearance, suggesting they contained a foreign substance, as commonly seen in corundum exposed to high-temperature heating. Also note the iridescent colours as seen in some natural sapphires.
EDXRF analysis
Qualitative EDXRF analysis revealed the presence of Ca, Ti, Fe, and Ni in all samples. The presence of Ti indicates that the sheen producing inclusions are most probably rutile; this probability becomes even stronger due to the fact that Ti was detected only when the stones were analysed from the table facets. Not Ti was detected when samples were analysed from the pavilion facets (which did not had such inclusions). Further, the greenish blue and green samples also revealed distinct Co peaks along with Cr (weak) and above mentioned elements.
Conclusions
Although identification of these sapphires is not so challenging, but a care has to taken while dealing in such stones. Two of the stones studied here readily displayed the identifying features like curved zones, but this may not appear so convincingly in lighter coloured samples like in yellows. Further, when presented in the routine trade, these may pose problems for the traders, who in general do not use a lens or any other gem testing equipment.
All photographs and photomicrographs by Gagan Choudhary