Spinel

Gemmology Queensland writes:

The Flawless Spinel

According to Scottish gemologist, Alan Hodgkinson, red spinels that are flawless, when observed in a horizontal microscope, are few and far between. But the proposal to a flawless red spinel is sound, as this forces gemologists to investigate further, a previous generation of gemologists did have to discover Plato structures in otherwise flawless Verneuil synthetic corundums.

Among the many gemological treasures in Anderson’s Gem Testing was his discovery that the multi-line emission spectra of natural spinel contrasted with the single line emission of the various synthetic spinels. Seeing is believing and as with most processes, technique is all important for success.

It is not easy to achieve positive results, when attempting to use longwave ultraviolet light to study the fluorescent emission spectrum of gemstones, for ultraviolet damage to eyes is a real possibility. Better results may be achieved using tungsten or halogen light that has been filtered with a blue filter.

The blue exciter filter, sold by McCrone Instruments, or a saturated solution of copper sulphate, is ideal for this purpose. When this filter is crossed with a red Barrier filter, it then becomes possible to observe fluorescent chromium spectra in gemstones.

The blue exciter filter can be used alone to detect the emission spectra of red spinels, however, a strong light source must be used to ensure success.

Preferably:
- work in total darkness.
- illuminate the spinel to achieve maximum transmission.
- place the blue exciter filter against the stone, on opposite side to the light source.
- observe the emission spectrum with a good quality spectroscope such as the OPL diffraction grating spectroscope. Note that the spectroscope should be held close to the blue filter so that it can analyze all the wavelengths emitted by the fluorescing stone.

What you observe from red spinel will be either the multi-emission lines of natural red spinel that are contrasted against the general gloom of the natural spinel, or only a single 686nm emission line that identifies the various synthetic red spinels.

Hodgkinson’s advice to those who wish to learn the fluorescent emission lines in red spinel is…..try, try, try again. Success will be your reward.