Spinel

Color due to chromium. Two fine lines and weak band in red. Very faint general absorption of the yellow and green

The cobalt spectrum produced by synthetic blue spinel is very similar to that seen in cobalt blue glass but with certain variations. The band in the green is centered at 540nm., the band in the yellow is centered at 580nm. and the band in the red is centered at 635nm. This gives the appearance of the three bands being closer together compared to the spectrum in blue glass. Note also that here the center band in the yellow is broader than the other two.

Color due to cobalt. As the cobalt content increases in synthetic blue spinel, the stone appears a deep cobalt blue and the spectrum shows broader and darker absorption bands. The one at 540nm. may merge with the one at 580nm. to form a wide dark area in the green. Here a faint narrow transmission window is seen at about 500nm.

By a slight alteration to the orientation of the stone, in relation to the direction of the light source, the full impact of the fluorescing emission lines can be seen. The strongest at 686nm. is supported by another eight lines, three on the long wave side and five more on the short-wave side

Color due to chromium. With the strong central absorption band eliminating all green and yellow light. Only the very hint of thin vague emission lines are detected, with difficulty, in the red and are easily missed in this transmitted absorption spectrum

The fluorescence spectrum of this spinel is more convincing than the absorption spectrum. Here we see the series of emission lines often referred to as the "Organ Pipe" pattern where the strongest line at 686nm. is prominently displayed. Paler red spinels may display this whole series of emission lines even more strongly. These emission lines can be detected as seen here in scattered light conditions

In the early Verneuil stones and in most of the modern flux grown stones, the fluorescent lines in the red are less easily defined and mainly dominated by the single line at 686nm

Color due to chromium. The absorption spectrum of lab Created red spinel is very similar to the natural counterpart with the main line in the red usually only detected as an emission line. The width of the central absorption band will vary according to the depth of color.

Color due to chromium. The broad absorption due to chromium in spinel is centered at 540nm. A little blue is transmitted but absorption takes over again from about 480nm. The lighter bright red spinel’s, lines in the red part of the spectrum are rarely detected in this dark red variety

Color due to chromiumIn this bright orange - red variety of spinel the doublet in the red at 685/684nm. may be seen but often difficult to detect as absorption lines. However, the absence of lines in the blue, and the broad band in the green being centered more towards the short-wave side at 540nm. than in ruby, (550nm.) is sufficient to distinguish between these two gemstones

Color due to chromium. Spinel in this particular pinkish red hue can easily be confused with a fine ruby. Three features in the spectrum can help to separate the two stones. The first is the absence of the lines in blue area. The second, perhaps less obvious, is the position of the central absorption band which is centered at 440nm. compared to 550nm. in ruby. Finally, in the absorption spectrum lines in the red are difficult to detect, but the bright emission line at 686nm., is noticeably further away from the long wave end compared to ruby at 693/694nm

Manipulation of the stone in a high intensity light can reveal several other emission lines in the red. Often referred to as an "Organ pipe" pattern .This can be detected to a limited degree by scattered light but is best seen under crossed color filters where the majority of the series of about ten lines may be seen. Only five are seen here due to the transmission of the accompanying orange-red light, one on the long wave side and three on the short-wave side of the strongest line at 686nm

Color due to cobalt. The spectrum of this Russian flux grown spinel shows an intense three band cobalt spectrum with a narrow transmission window in the green.

Color mainly due to iron. This spectrum from a dark blue spinel illustrates how absorption is stronger as the color becomes deeper and more saturated. The spectrum pattern is similar to a lighter blue, but here all the bands are now broader and darker especially the iron band at 460nm. Note there is very little transmission beyond 630nm.

Color due to iron. A slight green overtone in the light blue spinel that provided this spectrum is not unlike a sapphire but the single band in the blue is further to the long wave side at 460nm. as opposed to 450nm in sapphire. Two other broad diffuse bands can be seen centered at 560nm. and 585nm. then total absorption beyond 650nm.

Color due to cobalt and iron.This spectrum is from a less vibrant blue spinel but still has no green overtone due to the very weak iron line at 460nm. And the cobalt bands are weaker with less transmission in the deep red.

Color mainly due to cobalt. Spinel is seldom a fine blue comparable with sapphire and is usually seen with a dark green overtone due to ferrous iron. When a high cobalt content is present with very little iron, as in the spectrum of the stone shown here, a finer blue is achieved. In the spectrum the absorption line associated with ferrous iron at 460nm. is extremely faint. This and the three bands due to cobalt make it not unlike the spectrum seen in synthetic flux grown spinel but there would be no iron line at 460nm. in the synthetic.

Color due to cobalt and iron. The iron line at 460nm. is distinct but the stone is still a good blue color. The cobalt bands are present but there is little transmission beyond 650nm.

Color due to iron and cobalt. A very dark greenish blue spinel provides a spectrum with a strong iron band at 460nm. It also shows three cobalt bands in the green yellow and orange areas. Very little transmission is seen from 540nm. to 610nm. but note the two very narrow windows at 555nm. and 585nm.

Color due to iron. The spectrum of this almost colorless spinel with the slightest tint of lavender blue yields little to help identity. However, on careful inspection a very fine line can be seen at 460nm. in the blue. The two faint bands centered at 570nm. and 590nm. create a delicate balance which enables a slight color shift to a lilac pink in tungsten light.

Color due to chromium. Fine lines or bands in the red. General absorption in yellow and green. The width and darkness of the absorption bands varies. The number of lines seen in the red may vary, in some stones they may appear as a doublet or as emission lines. There are usually no lines visible in the blue