Color, spectra & luminescence Chapter4

Table 4.3: Fluorescent reactions of untreated corundums

Variety Long wave UV (366 nm) Short wave UV (253.7 nm)8 X-rays

Ruby (Including pink) • Burma, Sri lanka, Vietnam, • Moderate to extremely strong red • Moderate to strong red to orangy • Moderate to strong red

Afghanistan, Kenya, Tanzania to orangy red red • Thailand/Cambodia • Weak to moderate red to orangy • Inert to moderate red to orangy • Inert to moderate red

red red

Blue sapphire • Sri Lanka, Kashmir (India) • Inert to strong red to orange • Inert to strong red to orange • Dull red or yellowish orange • Australia, China, Colombia, Nige- • Inert • Inert • Inert

ria, Thailand, Cambodia

Purple & violet sapphire • Burma, Sri Lanka, Vietnam • Weak to strong red to orange red • Inert to strong red to orange red • Inert to strong red

Yellow & orange sapphire • Australia, Thailand • Inert • Inert to weak red • Inert • Sri lanka • Inert to strong orange • Inert to strong orange • Weak to strong orange • Vietnam • Not reported • Not reported • Not reported

Green sapphire • Australia, Thailand • Inert • Generally inert; rarely weak red • Inert

to orange

Colorless sapphire • Sri lanka • Inert to strong orange to red • Inert to moderate orange to • Inert to moderate red or orange

orange red

a. Heat treated gems often display a chalky blue-green fluorescence under SW. Generally it is the colorless areas of the gem which show this fluorescence. Dyed gems may display a fluorescence concentrated in the cracks (where the dye is located). This fluorescence may differ from that of the stone itself.

be found even in Sri Lankan sapphires where the 451.5 nm line is not present (Anderson & Payne, 1948). These W lines are absent in Verneuil synthetics. Nautiyal & Mukher-jee (1958) reported lines at 388 and 375 nm for natural sap-phires. As they used more sophisticated measuring apparatus, their wavelength measurements are probably more accurate than Anderson's.

W spectra have proven useful in separating natural corundums from a variety of synthetics. Unfortunately, the cost of W spectrophotometers is out of reach of most labs. However, Kriiss of Germany has manufactured a cheaper W spectroscope that will allow many separations to be made. The Kriiss WS 2000 makes use of a mercury emis-sion lamp which has a total of 17 emission lines ranging from about 260 nm in the UV to 580 nm in the visible region. Kriiss has numbered these lines, and those numbers are used in Table 4.2, which is based on Montgomery (1991b).

By use of a fluorescent screen, the UVS 2000 allows one to see whether a gem transmits light in the region from 260-580 nm. Any lines not visible indicate absorption of that area by the gem in question.

Infrared (IR) spectra Peretti & Smith (1993) reported that Russian hydrother-

mal synthetic rubies show a number of sharp lines between 3000 and 3800 wavenumbers, in addition to the normal infrared spectrum of corundum. Smith & Surdez (1994) found a number of sharp lines between 3100 and 3400 wavenumbers in Burmese rubies from Mong Hsu.

Pleochroism Corundum crystallizes in the trigonal division of the hexag-onal system. Lattice points are equally spaced along the hor-izontal plane, but differently spaced in the vertical plane. This type of symmetry in the distribution of electron clouds means that light will behave differently, depending on its direction of travel.

In corundum and other uniaxial gems, light entering in any direction except parallel to the c axis is split into two rays. Because of differences of atomic symmetry, and subse-quently, vibration direction, each ray may be absorbed dif-ferently. Thus, one ray takes on one color, while the other takes a different color. This difference in color with direction is termed pleochroism ('multicolored'). Uniaxial materials possess two vibration directions (ro and e), and so, poten-tially, two different colors, one corresponding to each vibration direction. As a result, uniaxial gemstones, such as corundum, are dichroic. Because pleochroism results from the difference in vibration directions, it varies in a manner similar to the refractive indices. 5 The color corresponding to the ordinary ray (o-ray) is constant throughout the crystal, while that corresponding to the ray (e-ray) is variable. Parallel to the c axis (optic axis), the e-ray color matches that of the o-ray. Thus no pleochroism is seen in this

5· Surprisingly, Denning & Mandarino ( 1955) found in a study of synthetic ruby that, while the pleochroic pattern was similar to the RI variation, it did not match exactly. In fact, for light of 486 nm, there was noticeable chroism even parallel to the c axis. But the above model is enough for our purposes.

RUBY & SAPPHIRE 8.0

Chapter4 Pleochroism

Table 4.4: Visible spectra of corundum

Variety Spectra description (from 400 to 700 nm)8

Ruby (including pink) • Broad absorption from -400-450 nm • Blue transmission from -450-500 nm • Narrow absorption lines at 468.5, 475 and 476.5 nm • Broad absorption band centered at about 550 nm. This band is stronger for the ordinary ray. b • Strong orange and red transmission from about 600 to the infrared • Narrow absorption lines in the red at 659.5 and 668 nm (weaker) and 692.8 and 694.2 (stronger). These may reverse into

fluorescent (emission) lines. Note: Fa-rich rubies may display Fe lines, in addition to the above. This has not been reported in synthetic corundum.

Blue sapphire • Slight absorption in the deep violet • Three lines of decreasing strength at 451.5, 460 and 470 nm; in Fa-poor stones, only the 451.5 nm line may be seen. The

lines are stronger for the ordinary ray. Only a weak 451.5 line has been reported in synthetic sapphires, and this is rare. Thus the full 3-line complex is proof of natural origin.

• Broad band of weak absorption centered about 560 nm in some deeply colored specimens. This has no diagnostic signif-icance, being seen in both natural and synthetic corundums. It is due to the ordinary ray.

• Crowningshield (1959) reported on a natural blue sapphire with the normal 451.5 nm line, but also with lines at 500 and 510 nm, and a fine line at 610 nm. This has never been reported elsewhere. Anderson (1980) reported that Verneuil syn-thetic blue sapphires may have even weaker bands on either side of that at 451.5 nm, which are nearly impossible to see. A vague blur is seen, or sensed, at about 490 nm, just where the green ends and the blue begins. The other is an even weaker blur in the violet at about 428 nm.

Note: Cr-rich natural or synthetic blue sapphires may display a weak Cr spectrum (fluorescent lines at the end of the red).

Violet/purple sapphire • Combination of the Fe and Cr spectra above. Only rarely has a weak 451.51ine has been reported in synthetic sapphires.

Yellow sapphire • Fe spectrum (see blue sapphire above); some gems may show a weak Cr spectrum • Heat-treated Sri Lankan yellows may show complete absorption from 400 to 450 or 500 nm. This is of no diagnostic value,

as it is also found in some Verneuil synthetic corundums. • On rare occasions, a weak line at about 455 nm has been reported in Verneuil synthetic yellow sapphire.

Orange sapphire • Combination of Fe and Cr spectra (see blue sapphire and ruby above) • Heat-treated Sri Lankan oranges may show complete absorption from 400-450 or 500 nm. This is of no diagnostic value,

as it is also found in some Verneuil synthetic corundums.

Green sapphire • Strong and complete Fe spectrum (see blue sapphire above). This is virtually always present in natural stones, but is not found in synthetic green sapphires.

• Some synthetic green sapphires show a line at 500, 530, 635 and 690 nm. This may be due to a combination of cobalt, vanadium and nickel.

Colorless sapphire • Generally not diagnostic; may display extremely weak Fe and/or Cr spectra

Color-change sapphire Vanadium spectrum (common in synthetic corundum; rare in natural corundum) • Broad absorption from 400-450 nm • Single absorption line at 4 73 nm • Broad absorption band centered about 690 nm • Str.ong transmission through the orange and red. A narrow fluorescent (emission) line may be seen at about 680 nm (this

gives the gem a change of color). Cr/Fe spectrum (common in natural and Verneuil synthetic corundum) • Combination of Fe and Cr spectra (see blue sapphire and ruby above)

a. All wavelengths are approximate only. b. Pleochroism can have a slight effect on the absorption spectrum. By rotating a polaroid plate over the spectroscope eyepiece, the spectra of both the ordinary and extraordi-

nary rays can be viewed independent of one another.

direction. In intermediate directions, as the direction of travel deviates from the c axis, the e-ray's color steadily diverges from that of the o-ray.The e-ray color reaches its maximum divergence when light travels perpendicular to the c axis. Thus the strongest pleochroism in uniaxial stones is seen at 90° to the optic axis.

Not all doubly refractive minerals display pleochroism, because, in some, the difference in absorption between the two rays is not enough to be detected with the eye. Most varieties of corundum do display strong pleochroism, but its strength may vary with the stone's depth of color. Generally, the greater the specimen's depth of color, the greater its ple-ochroism, and vice-versa. No pleochroism is observed in col-orless stones, or in colored stones when looking parallel to an optic axis.

Table 4.5 lists the pleochroic colors of different corundum varieties.

Pleochroism with the dichroscope To observe pleochroism, a dichroscope is used. This makes

use of either specially-configured polaroid or calcite. The cal-cite type is preferable because it is colorless, and so allows finer gradations of color to be distinguished. Furthermore, some of the polaroid types tend to go bad over time, losing their ability to show pleochroism.

Observing corundum's pleochroism with the dichroscope will reveal two colors in all directions except parallel to the optic axis, where only one is seen. The key is to examine the stone at 90° to the optic axis, where thee-ray reaches its max-imum divergence from that of the ordinary ray.

RUBY & SAPPHIRE 81