Following this discussion of the pocket-friendly gemmological instruments, part II continues to complete the field gemmology bag, while also beginning to stock the gemmologist’s office with some of the more intensive tools used in identifying gemstones.
The ultraviolet torch
Yes, ultraviolet – like that emitted by the sun! Gemmologists use both longwave ultraviolet light (LWUV, better known to the public as UVA) and shortwave ultraviolet light (SWUV or UVC) to observe photoluminescence.
More advanced gemmological instruments use even shorter wavelengths to produce more informative reactions.
When exposed to different types of ultraviolet light, some gemstones get excited on an atomic level and emit light to release this extra energy, known as photoluminescence.
This may be in the form of either fluorescence or phosphorescence.
The former is the immediate reaction observed when the ultraviolet light is hitting the gemstone, and the latter is the residual light the gemstone continues to emit after the ultraviolet light has been removed.
The various colours gemstones fluorescence or phosphoresce can provide information on the gemstone species, the gem’s country of origin, and even treatments it may have undergone.
Picture if you will, photoluminescence as a glow gemstone can exhibit in an array of fantastic colours – some peachy like in sapphire, others vibrant red like in ruby, or even different colours in geometric patterns like in diamonds.
Although testing for photoluminescence never provides enough information to identify a gemstone on its own, a strong LWUV torch like the Convoy torch is a great addition to any gemmologist’s tool bag.
Both SWUV and LWUV illumination can be offered in one instrument, such as in the Gemetrix PL-Inspector.
This is particularly handy when looking for clues to distinguish a natural diamond from a lab-created diamond, for example, as natural diamonds will fluoresce stronger in one type of UV than the other, and vice versa.
The polariscope
Though traditionally a more bench-top suitable instrument, the polariscope may also be hand-held or even homemade to be deconstructed and transportable.
The foundational concept behind the polariscope is the polarisation of light in a similar way to how polarised sunglasses work.
As it travels, white light is vibrating in all different directions. When passed through a polariser, all directions of light vibration are cancelled except one.
A polarising sheet at the bottom of the polariscope apparatus creates a single plane of vibrating light, only passing through in the given direction.
Another polarising sheet sits at the top of the apparatus, called an analyser, above the stage the gem would sit on.
These two polaroid sheets (top and bottom) are rotated so that the allowed path of light is at 90 degrees to each other and light passing through the bottom cannot pass through the top when no gemstone is in the middle.
When light passes through the gemstone on the stage between the polarisers, it bends and continues out (in singly refractive gemstones), or bends and splits into two rays (in doubly refractive gems).
If the gemstone observed is singly refractive, no light will make it through and the gem appears dark through a 360 degree rotation of the stage.
For doubly refractive gemstones, one of the bent rays of light is able to make it through in some rotations and the stone will change from appearing dark to appearing light as you rotate the stage it sits on.
Deeper understanding
So, what does this mean for the gemmologist?
A polariscope is a great tool for determining whether a gemstone is singly refractive or doubly refractive.
Doubly refractive gems include examples such as tourmaline, sapphire, and zircon. Singly refractive include spinel, diamond, garnet, to name a few.
This allows you to narrow down your options and step closer to determining which gemstone you may be dealing with.
More on how gemstones interact with light and how a gemmologist may use this to their advantage will be unravelled in part III of this series in the next issue of Jeweller.
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