A helpful test for yellow-colourless-light coloured sapphires
Identification of sapphire usually does not pose any problems and can be easily accomplished by standard instruments. However the separation of natural from synthetic counterpart sometimes become challenging when light coloured clean sapphires (yellow to colourless) are encountered.
The separation between the natural and synthetic sapphires (especially light coloured - colourless/ yellow) is commonly done using magnification features. Presence of gas bubbles / unmelted powder along with ‘plato lines’ are observed in synthetics while inclusions like crystals, fluid inclusions, zoning features, parting planes, etc. in natural sapphires. However, often there are cases when these stones are free of inclusions and hence the separation becomes lot more challenging and time consuming
There is a need for a quick and effective method of separation of natural and synthetic yellow and colourless sapphires. One such method is SWUV transparency; this method has been known for quiet sometime, but not commonly used by student gemmologists and is considered to be a laboratory’s instrument
What is SWUV Transparency?
SWUV transparency is different from UV fluorescence; certain materials are transparent to some specific kinds of rays while opaque to some. For example, flesh / muscles are transparent to X-rays while the bones are opaque. But the same is not true with visible light/eye; both appear opaque. Also, diamonds are transparent to X-rays but Synthetic Cubic Zirconia is not; but both are transparent to visible light. Hence, it is not necessary that the materials, which are transparent to visible light, are also transparent to all various types of rays. The same principle goes with SWUV. Some materials may be transparent to SWUV (like flame-fusion colourless synthetic sapphires) and some materials are opaque (eg. natural sapphires or other synthetics).
This method has been documented by Yu R.M. and Healy D (1980) “A Phosphoroscope”, Journal of Gemmology, Vol 17, No. 4, p.250 and revisited by Elen. S & Fritsch. E. in “The Separation of Natural from Synthetic Colourless Sapphire”, Gems and Gemology, spring 1999, p 30- 41. These articles have presented an instrument “Phosphoroscope” to check SWUV transparency of gems, which uses a ‘phospor’ plate as the base for stones and as the UV rays fall on this plate, it fluoresce strong white.
Instead of building a complex instrument or purchasing it, presented here is a crude way of checking SWUV transparency using a standard gemmological UV lamp coupled with cheap daily use scraps!!
What is required?
A SWUV lamp (with proper viewing cabinet and accessories like protective glasses, gloves etc.)
An ordinary cosmetic mirror
A (broken) piece of fluorescent day light lamp (tube light) (Keep the white powdery material intact on the surface of the broken piece of lamp as it consists ‘phosphor’)
How to do the test?
Place the UV lamp in a dark room preferably with proper viewing cabinet (else rest it such a way that the UV light falls below).
Hold the piece of broken tube light such that the powdery side is at top. When switched on, the piece should glow due to the white powder.
Place the stones table down on this glass piece with powder surface.
Now place the mirror below such that one can view the reflection of the stones on the mirror.
Result:
Samples that do not allow the UV radiation to pass through and hit the powder will appear opaque (figure 5, right), while samples which allow the rays to pass through them will appear transparent with the darker perimeter on the mirror (figure 5, left). This difference depends on the amount of impurities present in the sample under test. Hence, a synthetic sapphire, which is a pure form of Al2O3 will appear transparent and a natural sapphire which consist of some impurities of Fe, Ti, or Ga will appear dark. Similarly, hydrothermal or flux stones containing higher percentage of impurities will appear dark and cannot be differentiated from natural counterparts by this method. In addition, if the flame fusion stones have undergone some sort of treatment the results become unusable. It is always advisable to compare any unknown material with the known natural or synthetic samples at the initial stages.
While checking for light yellow/colourless sapphires:
If a dark spot is observed: Natural or Synthetic (not very helpful)
If a bright spot is observed: Synthetic (conclusion)
The point to remember is when a bright spot is observed, the sapphire is confirmed synthetic but when a dark spot is observed it may be natural or synthetic. Such a method of checking SWUV Transparency is very easy to construct, cheap and highly effective when used properly. However certain precautions are to be taken as follows:
Take precautions not to expose unprotected skin or eye to SWUV radiations. Use protective eyewear and gloves.
The broken piece of tube light may be sharp and dangerous. Be very careful and students are specifically advised to use, store and dispose off the pieces carefully.
Highly included and opaque stones may provide wrong results.
Always start with the known set of stones and compare the results with unknown samples. Do not skip the routine gemmological tests.
This method if applied properly and with care will prove to be a fast and effective method of confirming light coloured synthetic sapphires. If you have any problem in understanding the process or the principle, feel free to contact me or the Gem Testing Laboratory, Jaipur.
PS This article was submitted for the Lab Information Circular March 2008. To get the complete issue inform me or contact GTL, Jaipur. I acknowledge Gagan Sir, Khan Sir and Meenu Mam, GTL for their co-operation and support
