THE COLOR ORIGIN of GEM DIASPORE: CORRELATION to CORUNDUM Che Shen and Ren Lu

FEATURE AR ICLES

THE COLOR ORIGIN OF GEM DIASPORE: CORRELATION TO CORUNDUM Che Shen and Ren Lu

Color-change diaspore, known commercially as Zultanite, is sought by designers and consumers for its special optical characteristics, namely its color and color change. Understanding the color origin of gem-grade diaspore could provide a scientific basis to guide its gemological testing, cutting, and valuation. This study uses ultravio- let-visible (UV-Vis) spectra and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) to examine the color origin of color-change diaspore and to compare it with corundum. As Raman spectra vibration intensities are closely related to crystal direction for diaspore, crystal orientation was determined through Raman spectroscopy. The color correlation between color-change diaspore and corundum confirmed the identity of each chromophore. In addition, the effectiveness of different chromophores such as Cr3+ , Fe3+, Fe2+-Ti4+ pairs, and V3+ between gem-quality diaspore and corundum is compared quantitatively.

em-quality diaspore occupies an important structure consists of AlO octahedra (Hill, 1979; Lewis 6 position in the gem market due to its rarity, et al., 1982). Both types of crystals are composed solely Gstriking pleochroism, and color-change phe- of octahedral units. In addition, the diaspore structure nomenon (figure 1). The material’s value depends on is able to convert to corundum structure through de- these factors. A clear understanding of color origin hydration (Iwai et al., 1973). Due to their closely related offers considerable benefits for gemological testing, crystallographic structure and chemical composition, cutting, and even valuation of gem diaspore. By replacing the major elements in definite structural units through isomorphous substitution, trace elements play an important role in the color of gemstones. The AlO octahedra is a significant structural In Brief 6 unit that produces color when different trace ele- • Gem-quality diaspore with an attractive color and re- ments substitute for Al. For example, Cr3+ substi- markable color-change phenomenon is rare and often tutes for Al3+ in the AlO octahedra in jadeite and highly valued. 6 spinel, causing green and red color (Lu, 2012; Malsy, • The AlO6 octahedral structural unit, a very important 2012), while the substitution of Fe3+ for Al3+ in sap- structural unit related to gemstone color, is very similar phire produces yellow color (Emmett et al., 2003). between diaspore and corundum. Diaspore and corundum have a similar chemical • The structural similarity between diaspore and corun- composition and crystal structure (see figure 2). Dias- dum helps us confirm the color origin of diaspore pore, with the chemical formula AlO(OH), belongs to quantitatively. the orthorhombic space group 2/m 2/m 2/m (Hill, 1979); corundum, with the chemical formula Al O , 2 3 belongs to the trigonal space group 3 2/m (Lewis et al., we may speculate that there is also a close color corre- 1982). The crystal structure of diaspore̅ consists of lation. There are two other reasons for this hypothesis: AlO (OH) octahedra, whereas the corundum crystal 4 2 1. Compared to other gemstones containing the AlO octahedral structural unit, diaspore and 6 corundum share the closest similarity in average Al-O bond length, octahedral volume, and See end of article for About the Authors and Acknowledgments. degree of distortion of octahedral sites (quanti- GEMS & GEMOLOGY, Vol. 54, No. 4, pp. 394–403, http://dx.doi.org/10.5741/GEMS.54.2.394 fied by determining the octahedral quadratic © 2018 Gemological Institute of America elongation). For specific data, refer to table 1.

394 COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 Figure 1. This 24.60 ct color-change diaspore is shown under D65 illumination (left) and A illumination (right). Photos by Robert Weldon, courtesy of Zultanite Gems LLC.

2. The structural similarity between corundum (formerly Burma) and Turkey—and comparing them and diaspore was first referred to by Deflandre with high-quality natural and synthetic corundum. (1932). Both diaspore and corundum are con- The research aims to use the color correlation be- nected by octahedral units, and both contain tween diaspore and corundum to confirm the color edge-shared octahedra. However, there are face- origin of gem diaspore. shared octahedra along the c-axis that only exist in corundum, and this is the main difference be- MATERIALS AND METHODS tween the two. Other related studies involving Samples. Nine diaspore samples from Turkey were X-ray powder photography (Francombe and purchased from a gem dealer at the Beijing Interna- Rooksby, 1959), single-crystal X-ray techniques, tional Jewelry Show in November 2016. Forty slices scanning electron microscopy (SEM), and high- of diaspore were collected in the field from Mogok, temperature optical microscopy (Iwai et al., Myanmar, by author RL. All samples were prepared 1973) have confirmed a structural similarity be- as wafers. These wafers are perpendicular to the a- tween the two gem materials. axis, b-axis, and c-axis, respectively. Photos of sam- This intensely close relationship between corun- ples are shown in table 2. dum and diaspore with respect to the octahedral Al site and their overall structural similarity indicates Raman and Photoluminescence (PL) Spectroscopy. that chromophores substituted into the Al site of both Raman and PL spectra were collected from the sam- materials are expected to present similar UV-visible ples with a Renishaw inVia Raman microscope sys- absorption features. Consequently, this study adopts tem at the Gemstone Center of Hebei GEO an analogy to address the color origin of diaspore by University in China. Raman spectra were collected quantitatively analyzing the trace-element chemistry from 100 to 1400 cm–1 using Nd-YAG laser excita- in diaspore from two geographic origins—Myanmar tion, producing highly polarized light at 532 nm

TABLE 1. Crystallographic parameters of corundum, diaspore, and spinel.

Average Octahedral Deviation of average Al- Deviation of octahedral Deviation of quadratic Octahedral Gemstone distance of quadratic O distance (% difference volume (% difference elongation (% difference volume (Å3) Al-O (Å) elongation from corundum) from corundum) from corundum) Corundum 1.9133 9.0754 1.0202 0 0 0 Diaspore 1.9154 9.1042 1.0203 0.10976 0.31734 9.8020 × 10–3 Spinel 1.9327 9.4955 1.0092 1.0140 4.6290 1.0782

Note: The crystallographic parameters are from Lewis et al. (1982), Hill (1979), and Peterson et al. (1991).

COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 395 Figure 2. These views of the crystal structure of diaspore (top) and corundum (bottom) illustrate the basic building blocks, AlO (OH) and AlO 4 2 6 octahedra, and their geomet- ric relationships. The crystallographic orientation is shown in the upper left corner projected down the c-axis. a The blue spheres in the diaspore’s crystal structure represent the hydrogen atoms. The structure is characterized by edge-shared octahedral dou- c b ble-chains, which are ex- tended along the a-axis. The corner-linked octahedral layers are connected along the a- axis. In addition, the b t OH-vector is approximately parallel to the a-axis. In the crystal structure of corun- c dum, each octahedron is edge-shared, with three octahedra in the a-b plane. All oc- t tahedra are face-shared, with one octahedron along the c- b t axis. The letters b, c, and t t represent the different layers in corundum (bottom, center, \_ and top). Based on Hill (1979) for diaspore and Lewis et al. c a (1982) for corundum.

(1800 lines/mm grating, 10 s). The orientation of all with 27Al using a fixed internal standard method. We samples was carefully controlled. The polarization selected three spots on each sample for general chem- direction of the injected laser was parallel to the a- ical composition. axis, b-axis, and c-axis. The 600–800 nm spectral ranges for Raman photoluminescence analysis (under UV-Vis Spectroscopy. Five diaspore samples (Dia-006- liquid nitrogen conditions) were excited by an Nd- a, Dia-006-b, Dia-006-c, Dia-Bur-001, and Dia-Bur-002) YAG laser at 532 nm (1800 lines/mm grating, 10 s). were prepared as well-polished, oriented wafers with various thicknesses. UV-Vis spectra were collected LA-ICP-MS Analysis. For trace-element analysis, we with a Perkin-Elmer Lambda 650 UV-Vis spectropho- used an Agilent 7900a ICP-MS coupled with a deep- tometer equipped with mercury and tungsten light UV laser at 193 nm excitation at the State Key Lab- sources and photomultiplier tube (PMT) detectors in- oratory of Geological Processes and Mineral stalled in an integrating sphere. The spectra were col- Resources, China University of Geosciences in lected at the Gemological Institute, China University Wuhan. NIST glass standard SRM 610 and USGS of Geosciences in Wuhan. Polarized spectra were col- glass standards BHVO-2G, BIR-1G, and BCR-2G lected in the 300–800 nm range with 1 nm spectral were used for external calibration. Ablation was resolution at a scan speed of 267 nm/min. The spectral achieved using a 44 μm diameter laser spot size, a baseline was corrected by subtracting spectral offsets fluence of around 5 J/cm2, and a 6 Hz repetition rate. at or beyond 800 nm, where the chromophores’ fea- The diaspore was initially standardized internally tures were insignificant or nonexistent.

396 COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 TABLE 2. Trace-element composition of diaspore by LA-ICP-MS (in ppma), with standard deviation in parentheses.

Dia-006 Dia-008 Dia-Bur-001 Dia-Bur-002 Trace element, with detection limit

Ti—9 110 (3) 185 (0.9) 220 (13) 418 (5) V—0.1 6 (0.2) 4 (1) 44 (2) 359 (2) Cr—1 36 (0.7) 32 (2) 2166 (58) 2081 (29) Fe—20 1169 (12) 548 (53) 4 (bdl) 1 (bdl) bdl = below detection limit

RESULTS AND DISCUSSION ented sample from Turkey, the Burmese diaspore Chemical Analysis. Table 2 shows the chemical could be oriented properly. Raman spectra of both the composition (expressed in ppma) of two diaspore Burmese and Turkish diaspore were collected in the samples from Turkey (Dia-006, Dia-008) and two 100–1400 cm–1 range. from Myanmar (Dia-Bur-001, Dia-Bur-002). We con- According to previous research by Ruan et al. cluded that the Burmese samples contained more Cr, (2001) and San Juan-Farfán et al. (2011), the band at while the Turkish diaspore had a higher Fe content. 154 cm–1 is assigned by the rotation of two edge- shared AlO octahedra around the c-axis. The most However, the V content of the Burmese samples was 6 higher than that of the Turkish samples, especially intense peak, at 447 cm–1, is related to Al-O symmet- in Dia-Bur-002 (358 ppma). ric stretching modes, which means the different polarization direction of the laser has little impact on Raman Spectroscopy. Unlike Turkish diaspore, this peak. Therefore, we normalized the peak inten- Burmese material often occurs as thin crystals that sity at 447 cm–1 and compared the intensity at 154 are difficult to orient by crystal growth characteris- cm–1. When the polarization direction of the injected tics. However, Raman vibration is closely related to laser is parallel to the c-axis, the intensity of the 154 crystal direction (see http://rruff.info/diaspore/ cm–1 peak is much stronger than those that run par- display=default/R060287). By comparing the Raman allel to the b-axis and a-axis (see note on figure 3). spectra of the diaspore from Myanmar with the ori- Based on figure 4, we can accurately diagnose the

RAMAN SPECTRA

–1 447 cm Al-O E || a E || b 200000 E || c Figure 3. The Raman spectra of the Turkish 150000 diaspore, with the polarization direction of

the injected laser paral-

100000 of two edge-shared rotation

–1 lel to the c-axis (blue octahedra around the c-axis the c-axis around octahedra

6 trace), the b-axis (or- AlO 154 cm 154 INTENSITY (COUNTS) (COUNTS) INTENSITY 50000 ange trace), and the a- axis (green trace).

0 200 400 600 800 1000 1200 1400 RAMAN SHIFT (cm–1)

COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 397 RAMAN SPECTRA

447 cm–1 Al-O 120000 E || a E || c 100000 Figure 4. The Raman spectra of Burmese di- 80000 c aspore with the polar-

a 5 mm ization direction of the 60000

rotation of two of two edge-shared rotation injected laser parallel –1

octahedra around the c-axis the c-axis around octahedra to the short side, or a- 40000

6 axis (orange trace), and AlO 154 cm 154 INTENSITY (COUNTS) (COUNTS) INTENSITY 20000 the c-axis (blue trace).

0 200 400 600 800 1000 1200 1400

RAMAN SHIFT (cm–1)

crystal orientation by Raman vibration (154 cm–1). (E vector) is parallel to the a-axis, diaspore shows UV- The Burmese diaspore wafers were manufactured visible absorption features similar to those of corun- perpendicular to the b-axis based on perfect cleavage dum (see figure 5). Our corundum samples included along the {010} direction. Hence, we can consider the three synthetic sapphires—a Cr-bearing synthetic long side of the wafer parallel to the c-axis, and the ruby, a Fe-Ti bearing synthetic blue sapphire, a V- short side parallel to the a-axis. bearing color-change sapphire—and one natural yellow Fe-bearing sapphire from Garba Tula, Kenya. UV-Vis and PL Spectroscopy of Cr3+. Corundum, a In order to make our initial comparisons between uniaxial crystal, shows dichroism. Its UV-Vis spectra chromophores in corundum and diaspore, we have are often collected from two orthogonal orientations chosen to compare the spectra of corundum col- with polarized light (o-ray and e-ray). As a biaxial lected from the o-ray (R. Lu, previously unpublished crystal, diaspore shows trichroism; its UV-Vis spectra data) with those of diaspore collected from the ori- are collected from three orthogonal orientations with entation of polarized light parallel to the a-axis. polarized light (E a, E b, and E c). In gem cutting, From LA-ICP-MS data, we know that the Cr content the table of a rubyǁ or sapphireǁ is ǁ generally perpendi- is significantly higher in Burmese samples than in cular to the c-axis. When the polarization direction Turkish samples. However, as the Fe content is near

Figure 5. The three polar- UV-VIS SPECTRA izations of diaspore (E a, E b, E c) were collectedǁ

) 6 Sapphire: O-ray—1000 ppma Fe toǁ compareǁ with the spec- –1 377 Dia-006: E || b—1000 ppma Fe tra of sapphire (o-ray).

5 388 Dia-006: E || c—1000 ppma Fe Peaks at 377 and 388 nm Dia-006: E || a—1000 ppma Fe in sapphire and at 384 and 4 398 nm in diaspore are at- 398 tributed to Fe3+. The height 3 384 of sapphire’s 377 and 388 450 nm peaks (384 and 398 nm 2 448 in diaspore) is closely related to directionality. In 1 diaspore, the best match

ABSORPTION COEFFICIENT (cm COEFFICIENT ABSORPTION with the spectrum of 0 corundum (o-ray) is shown 350 400 450 500 550 600 650 700 750 when the polarization di- WAVELENGTH (nm) rection is parallel to the a- axis (E a). ǁ

398 COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 BOX A: SPECTRAL FITTING BASED ON THE BEER-LAMBERT LAW

There are additive properties that exist in a UV-Vis spec- • A is absorbance trum. A simple mathematical step will lead to the fol- • l is the path length of the light beam through the lowing equation: material sample • is the molar attenuation coefficient or absorp- A = A + A + … + A εi 1 2 n tivity of the attenuating species i in the material where: sample • A is total absorbance of all absorbers • c is the concentration (in ppma) of the attenuat- i • A , A , … , and A are the absorbance of each ab- 1 2 n ing species i in the material sample sorber n Based on this law, if we have collected the absorption Figure A-1 illustrates how this is accomplished. spectrum of a chromophore at a certain thickness, we According to the Beer-Lambert law, the absorption is can simulate absorption spectra of different concentra- proportional to the concentration of absorbers (chro- tions and thicknesses within a certain range. Figure 6 mophores) through which light passes. It can be expressed shows two spectra of Burmese diaspore. By subtracting simply by the following mathematical expression: the two spectra, we will essentially obtain only the V ab- n sorption since the Cr concentrations are almost the A=1 c same. Since we have the pure V3+ spectrum, it is easy to ∑ εi i

i=1 obtain the pure Cr3+ spectrum by subtracting the pure where: V3+ spectrum from the spectrum of Burmese diaspore.

UV-VIS SPECTRA Figure A-1. The absorption of violet sapphire (violet dotted line) shown here Violet sapphire (Cr: 1000 ppma; Fe2+-Ti4+: 60 ppma) O-ray; 1 mm thick represents the sum of the other two 3.0 spectra: ruby (red line) and blue sap- Ruby (Cr: 1000 ppma; Fe2+-Ti4+: 0 ppma) Violet sapphire Blue sapphire O-ray; 1 mm thick phire (blue line). The color circles cal- L*: 36 a*: 33 b*: –52 L*: 58 a*: 10 b*: –47

2.5 2+ 4+ culated under D55 illumination are - - Blue sapphire (Cr: 0 ppma; Fe -Ti : 60 ppma) O-ray; 1 mm thick demonstrated for all of the spectra. We 2.0 may conclude that violet is a combina- 0 ,.... , Ruby , ' tion of Cr3+ red with Fe2+-Ti4+ blue. As L*: 62 a*: 49 b*: –29 ,, '' 1.5 , described above for the additive properties, the absorption of violet sapphire

ABSORBANCE ABSORBANCE 1.0 (A) is calculated from two known sets: A (the absorption of ruby) and A (the 1 2 0.5 absorption of blue sapphire). However, we could also use the difference (A–A ) 1 0.0 of any two spectra to show the remain- 300 400 500 600 700 800 900 ing absorption (A ). 2 WAVELENGTH (nm)

zero in the Burmese samples, we disregarded the ab- the author’s color analysis, chromium will cause a sorption of Fe in the UV-Vis spectra of that material. slight color-change phenomenon in diaspore (figure The spectra of Dia-Bur-001 and Dia-Bur-002 are 7). Additionally, we used the PL spectra (figure 8) to shown in figure 6. Although the Cr content of the confirm that gem diaspore and ruby show compara- two samples is almost the same (2166 and 2081 ble fluorescence spectra (694/693 nm for ruby and ppma, respectively), there is a large difference in the 690/693 nm for diaspore). absorption intensity. This difference is related to the absorption of vanadium. By subtracting the two UV-Visible Spectroscopy of Fe3+ in Diaspore and Sap- spectra, we essentially obtain only V absorption, phire. The UV-Vis spectrum of Turkish diaspore (fig- which can be subtracted from the other spectra to ure 9) shows features corresponding with those of obtain the pure Cr spectrum (see box A). Based on Fe3+-bearing yellowish sapphire from Garba Tula,

COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 399 ) UV-VIS SPECTRA PL SPECTRA –1 150000 Dia-Bur-002: E || a—2081 ppma Cr, 359 ppma V 30 Dia-Bur-001: E || a—2166 ppma Cr, 44 ppma V spinel 100000 20

ruby 10 50000

0 (COUNTS) INTENSITY diaspore 0 350 400 450 500 550 600 650 700 750 800 680 685 690 695 700 ABSORPTION COEFFICIENT (cm WAVELENGTH (nm) WAVELENGTH (nm)

Figure 6. The UV-visible spectra of the Burmese dias- Figure 8. The PL spectra of natural ruby and one of pore samples show a large difference in absorption the Burmese diaspore samples show very similar intensity. characteristics based on table 1. The crystallographic parameters of diaspore are very close to those of ruby, but quite different from those of spinel. The PL spec- Kenya. In those sapphires, the peak at 388 nm is at- tra of spinel (data provided by Dr. Chengsi Wang) are tributed to Fe3+, while the peaks at 377 and 450 nm considerably different from those of ruby and diaspore. Therefore, we can also use PL spectra to demon- are attributed to Fe3+-Fe3+ pairs (Ferguson and Field- strate that the structural units of diaspore and ruby ing, 1971, 1972; Krebs and Maisch, 1971). The spectra are very closely related. of diaspore show features very similar to those of Fe3+-bearing sapphire. Hence, the peaks at 384 and 448 nm are attributed to Fe3+-Fe3+ pairs, and the peak at 398 nm is attributed to Fe3+. diaspore show an obvious absorption band at around 570 nm. Based on the LA-ICP-MS data, chromium and UV-Visible Spectroscopy of Fe2+-Ti4+ Pairing in Dias- vanadium are very low in Turkish diaspore. In blue pore and Sapphire. The UV-Vis spectra of the Turkish sapphire, the absorption at around 580 nm is attributed

Figure 7. The calculated UV- UV-VIS SPECTRA visible spectra of diaspore

50 and synthetic ruby—both Diaspore: E-ray || a—1000 ppma Cr only containing Cr—have 45 0 0 Ruby: O-ray—1000 ppma Cr similar absorption regions Diaspore (D65) Diaspore (A) L*: 67 a*: 4 b*: 6 L*: 67 a*: 15 b*: 4 but show much different in- ) 40 –1 tensities. The color circles above the spectra, calculated 35 under D65 and A illumina- Ruby (D65) Ruby (A) tion, demonstrate coloration 30 L*: 35 a*: 68 b*: –22 L*: 42 a*: 72 b*: –14 -- 1.2 for diaspore and ruby with 1000 ppma Cr concentra- 25 1.0

tions for a 5 mm path length. 0.8 20 (The spectra of 1000 ppma 0.6 and 5 mm path length are 15 calculated rather than taken 690 692 694 696 698 700 directly from the sample. A ABSORPTION COEFFICIENT (cm COEFFICIENT ABSORPTION 10 similar method is used in the analysis of the other 5 chromophores.) According to

0 the author’s calculation, the

350 400 450 500 550 600 650 700 750 800 chromophore effectiveness of Cr3+ in ruby is about 5–10 WAVELENGTH (nm) times higher (peak intensity) than in diaspore.

400 COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 Figure 9. The UV-visible UV-VIS SPECTRA spectra of Turkish diaspore and yellowish sapphire Dia-006: E-ray || a—1000 ppma Fe from Garba Tula, Kenya,

) 6

–1 Sapphire: O-ray—1000 ppma Fe exhibit closely matched ab- 377 sorption features, especially 5 388 when polarized light is parallel to the a-axis. The color 4 0 circles are calculated under Diaspore (L*: 90 a*: –4 b*: 16) D55 illumination, demon- strating the coloration for 3 384 398 450 diaspore and sapphire with 1000 ppma Fe concentra- 2 0 tions for a 5 mm path 448 Sapphire (L*: 98 a*: –7 b*: 29) length. According to the au- 1 ABSORPTION COEFFICIENT (cm COEFFICIENT ABSORPTION thor’s calculation, the chromophore effectiveness of 0 Fe3+ in sapphire is about 1.6 400 450 500 550 600 650 700 750 800 times higher (peak inten- WAVELENGTH (nm) sity) than in diaspore.

to an Fe2+-Ti4+ pairing (Emmett et al., 2003). It is possi- tends to the near-infrared region, so the transmission ble that the absorption at around 570 nm in diaspore rate of red areas and green areas is not relatively equal. is related to Fe2+-Ti4+ (see figure 10). In blue sapphire, But in diaspore, these transmission areas are fairly the absorption of the Fe2+-Ti4+ pair does not cause the equal. The value of a* has changed from –3 to 1, which color-change effect because the absorption band ex- means the color will change from green to red.

Figure 10. The UV-visible UV-VIS SPECTRA spectra of Turkish diaspore and blue synthetic sapphire (containing Fe2+-Ti4+ only). 30 Dia-006: E-ray || a—50 ppma Fe2+-Ti4+ Sapphire: O-ray—50 ppma Fe2+-Ti4+ The two spectra have similar 28 Sapphire (D65) Sapphire (A) absorption regions but show L*: 10 a*: 26 b*: –71 L*: 5 a*: 22 b*: –75 26 much different absorption in- -- tensities. The color circles,

) 24 –1 0 0 calculated under D65 and A 22 Diaspore (D65) Diaspore (A) illumination, demonstrate L*: 95 a*: –3 b*: 8 L*: 96 a*: 1 b*: 7 20 Calculated for 50 ppma the coloration for sapphire Fe2+-Ti4+ pair with 5 mm path and diaspore with 50 ppma 18 Fe2+-Ti4+ pair concentrations 16 0 0 for a 5 mm path length. Since 14 Diaspore (D65) Diaspore (A) the color swatches of dias- L*: 77 a*: –7 b*: 31 L*: 78 a*: 6 b*: 27 pore calculated using 50 12 Calculated for 250 ppma 2+ Fe2+-Ti4+ pair with 5 mm path ppma concentration of Fe - 10 Ti4+ pairs do not show a clear 8 0.25 color change, we must calcu- ABSORPTION COEFFICIENT (cm COEFFICIENT ABSORPTION 6 0.20 late the colors, which are

0.15 shown in the spectra, at a 4 higher concentration of Fe2+- 500 540 580 620 2 Ti4+ pairs (250 ppma) for a 5

0 mm path length. The chromophore effectiveness of the 350 400 450 500 550 600 650 700 750 800 Fe2+-Ti4+ pair is about 50 WAVELENGTH (nm) times higher (peak intensity) in sapphire.

COLOR-CHANGE DIASPORE GEMS & GEMOLOGY WINTER 2018 401 Figure 11. The UV-visible UV-VIS SPECTRA spectra of diaspore (after

40 calculation) and synthetic Diaspore: E-ray || a—500 ppma V sapphire (containing V Sapphire: O-ray—500 ppma V

) 35 only). The two spectra –1 Diaspore (D65) Diaspore (A) show similar absorption L*: 44 --a*: 57 b*: –34 L*: 41 a*: 48 b*: –35 = 30 regions but different absorption intensities. The 25 0 0 color circles are calculated Sapphire (D65) Sapphire (A) under D65 and A illumina- 1.4 20 L*: 82 a*: –10 b*: 22 L*: 82 a*: 5 b*: 14 .,...···1.2 tion and demonstrate col- ...... ·· 1.0 oration for sapphire and .,...... 15 0.8 diaspore with 500 ppma V 0.6 0.4 concentrations for a 5 mm 10 path length. According to 500 540 580 600 the author’s calculation, ABSORPTION COEFFICIENT (cm COEFFICIENT ABSORPTION 5 ...... - the chromophore effective- ...... _...... --- ness of V3+ in diaspore is 0 350 400 450 500 550 600 650 700 750 800 about 2–7 times higher (peak intensity) than in WAVELENGTH (nm) sapphire.

Emmett et al. (2017) proposed that Si plays a role thetic sapphire. They also show similar absorption in the color chemistry of corundum. If corundum con- characteristics in their UV-Vis spectra, as both peaks tains Ti, Si, Mg, and Fe, the Fe will pair with Si before are at around 400 nm and 560 nm in diaspore and Ti. If diaspore is similar to corundum, Fe will charge- sapphire (see also figure 11). compensate Si before Ti. However, the instruments used for LA-ICP-MS show significant interferences for CONCLUSIONS the three silicon isotopes (Shen, 2010), and as a result In color-change diaspore, Cr3+,V 3+, and Fe2+-Ti4+ pairs the Si content is higher than the value shown. The au- are the chromophores, and all of these contain an ab- thor does not take Si into consideration because of this sorption area at around 560–580 nm. These chro- instrumental error, and assumes that all of the Ti pairs mophores may all play a role in causing the with Fe. Therefore, the actual content of the Fe2+-Ti4+ color-change effect. Raman spectroscopy proved to pair may be lower than the content determined be a powerful tool for determining the crystal orien- through analysis. tation of the Burmese diaspore wafer; it is also useful to measure and compare the directional UV-Vis spec- UV-Visible Spectroscopy of V in Diaspore and Sap- tra of diaspore and corundum. Because of their struc- phire. In natural corundum, vanadium content is gen- tural similarity, we can confirm the color correlation erally very low. Synthetic sapphire, which has been between corundum and diaspore and compare the ef- manufactured with V, will show an obvious color- fectiveness of the chromophores. According to the change effect. This type of synthetic sapphire is simi- calculation, the chromophore effectiveness of Cr3+, lar to natural alexandrite. However, there is some V Fe3+, and Fe2+-Ti4+ in corundum is about 5–10, 1.6, and in natural diaspore from Myanmar, which plays an 50 times higher, respectively, than in diaspore. How- important role in color origin and color change. ever, the chromophore effectiveness of V3+ in dias- We compared the spectra of diaspore (containing pore is approximately 2–7 times higher than in V only; see box A) to pure vanadium-bearing syn- corundum.

ABOUT THE AUTHORS ACKNOWLEDGMENTS Mr. Shen is a graduate student at the Gemmological Institute, The authors would like to thank Mr. Yungui Liu at HGTC (Gem- China University of Geosciences in Wuhan. In 2018, he was a stone Center of Hebei GEO University) provided Raman and PL research intern at the Gemological Institute of America in Carls- testing. Dr. Aaron Palke (GIA, Carlsbad) and anonymous peer bad, California. Dr. Lu is a professor at the Gemmological Insti- reviewers provided many helpful comments and suggestions. tute, China University of Geosciences in Wuhan.

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