A Blue Sapphire
Looking for stones to increase my reference library, this blue sapphire was purchased in the UK on e-bay and came with the following description;
“Really beautiful royal blue, similar to Ceylon, with no inclusions, Created sapphire. Not like common lab made material, this is grown through complex process resembling natural sapphire gems. This fiery oval multi-facet cut SAPPHIRE center stone will compliment all high quality jewelry setting.”
Size:4 CT, 10×8 mm HIGH LUSTER. Transparent ROYAL BLUE crystal
A “created” or “synthetic” gem stone is one which is cut from a man made crystal but in all other aspects is the same as the real thing. A created diamond, would be one made of carbon, and have the same optical and physical characteristics of the real thing. Cubic Zircona or CZ is not a “created” gem stone but a “stimulant”. It is not made of carbon and does not bend or refract light to the same degree as a diamond.
When I received the stone the first thing that struck me was the depth of the cut. So deep in fact that this stone could well prove hard to set and would ride very high. The actual dimensions were 10.12mm x 8.11mm x 6.18mm (LxWxD) . The girdle was thin and from the bottom of the girdle to the table barely measured 1.3 mm or 21% with the remaining 79% below the stone. The word “doublet” immediately sprang to mind. Doubleting is an old technique of literally gluing a sliver of good quality gem stone material to some thing cheap, often colored glass. With the exception of opals, this is not particularly common technique these days, although in the past it was very common to find ruby doublets. However the shape of this stone was characteristic of a doublet and the technique would be in line with the description “complex process resembling natural sapphire gems”.
Examining the stone, further through a dark field loop it appeared clean, but with some slight damage to the pavilion facet edges near the culet. These scratches appeared white! Scratches on the girdle however were a consistent blue. One thing was clear, there was more to this stone than met the eye.
When investigating gem stones, there are two characteristics that can be used in combination to eliminate the vast majority of possibilities; Specific Gravity (SG) and Refractive Index (RI). Specific Gravity is the ratio of an object weighted first in air and then is water. Specific Gravity The Refractive Index can loosely be described as the amount that light bends or changes direction when it passes from one medium into another, in our case from air into crystal.
The stone weighed 4.06ct in air and 3.05ct in water giving us an SG of 4.020 whi is consistent with corundum. RI reading proved to be 1.768 on the table and 1.765 on the pavilion facets, also consistent with corundum. The reading on the facets demonstrated that if it was a doublet it was corundum on corundum.
Sapphires and Rubies are both a crystalline form of aluminum oxide known as Corundum. Pure Corundum is clear and is some time knows as white sapphire. The color comes from impurities trapped in the crystalline structure. In the case of Ruby it is chrome that gives the red color. The range of colours in which sapphires are found are caused by differing impurities including iron in various forms, vanadium, titanium as well as several others. In blue sapphire its iron the causes the blue. A Ruby is in fact a fully saturated red sapphire!
Since the stone is not red, and it is yet consistent with corundum for both RI and SG, we can conclude sapphire. Purists will hold up their hands is horror at my drawing this quick conclusion , but I am not going to go into the details of optic axis, optic sign and pleochrosium, rest assured they were considered at the time.
I had paid less than a pound ($2) for this stone so if this stone was sapphire it had to be Verneuil since no other process is so cheap.. The Verneuil process causes characteristic curved growth lines or bubbles within the crystal, such as can be seen in this picture of a vanadium dopped corundum. In blue sapphires however, these can be difficult to see without an immersion microscope. Verneuil could also explain the heavy cutting, ie get the weight u and therfor increase the value. So we have a blue sapphire probably Verneuil but still no explanation for the white scratches at the culet.
The color given off by a stone is a mixture individual colors. These colors correspond to the elements contained within the chrystal&s structure and can provide a signature for both the element concerned and the gemstone. Using a prism or spectrometer to split the colors we find the spectrum is incomplete. A series of absorption bands, which appear as dark lines or gaps can be seen. In addition to absorption bands, there are emission bands which correspond to distinctly bright areas in the spectrum that provide the dominant color of the stone, as well as one of two surprises.
One such surprise can more easily be seen using a Chelsea Filter. The Chelsea Filter, a simple sheet of plastic, whilst predominantly green, also allows the transmission of deep red wavelengths around 690 nm. The presence of either Both Chrome or Cobalt can cause a red fluorescence to be emitted when the stone is illuminated by a white light that contains some ultraviolet. In the case of corundum Chrome would make it a ruby, so this test is really to look for Cobalt which is used in the Cobalt Diffusion method of manufacturing blue synthetic sapphire. In this case no such red glow could be found, thus eliminating this possibility. The spectrometer also failed to show the Cobalt characteristic triple band in the blue.
Frustration or inspiration, I don´t know. When making wine, adding sugar to water, fruit juice caused an increases in the specific gravity of the liquid. Further with respect to liquids specific gravity proportional to Refractive Index. So taking some boiling water and I stirred into it as much sugar as is could be persuaded to dissolve and set it aside to cool. This stuff was like syrup! A drop placed on my refractometer show an RI 1.45.
The shock came when the stone was viewed from the side whilst immersed in the syrup. The “blue” only ran down about 1mm below the girdle and phased out, not a sharp line like a doublet but just faded out. It looked like it had been “colored” from the table end and that was as far as the coloring process had penetrated. Coatings barely penetrate the surface, to achieve this level of penetration would require Cobalt Diffusion, but that had already been eliminated by the lack of red fluorescence.
In optics, the Critical Angle is the greatest angle at which a ray of light, traveling in one transparent medium, can cross the boundary with a second medium of lower RI without being totally reflected back. This now explained the cut of the stone and the strange white appearance of the scratches. What had been happening was that all light that had been entering the white part had been reflected though the blue, and conversely all the light that was allowed to pass out through the white has already passed through the blue! It was only by changing the critical angle using the syrup that this had now become visible. Some very clever cutting indeed.
Normally with Verneuil one would expect the color to be consistent. So why had this been done? Was it intentional? A couple of months later talking to a friend for the Asian Institute of Gemological Sciences, I discovered that he too had found this same phenomenon and that they had eventual tracked it down to a “bad” batch of Verneuil Boules that had been circulated. It seemed that the process had not been controlled properly and the iron oxide Fe2O3 had not mixed properly leaving the color on the outside and the center clear.
Not a doublet, and defiantly not resembling natural sapphire gems”!
“when you have excluded the impossible, whatever remains, however improbable, must be the truth.” – Sherlock Holmes