Editors Thomas M. Moses | Shane F. McClure Graphite Inclusions Forming specific growth episode when oxi- dized fluids rich in CO and H O Octahedral Outline in DIAMOND 2 2 Primary diamond deposits are usually passed through diamond bearing hori- found in mantle-derived igneous zons. This also explains the shift of rocks, with the principal hosts being the G band to a higher energy, due to kimberlite and lamproite. During the the higher strain near these newly ascent to the earth’s surface, dia- formed secondary graphite crystals (J. monds may be converted, partially or Hodkiewitcz, “Characterizing graph- entirely, to graphite and chemically ene with Raman spectroscopy,” dispersed and eliminated (A.A. Application Note: 51946, Thermo Fisher Scientific, Madison, Wiscon- Snelling, “Diamonds – Evidence of sin, 2010). Late formation of addi- explosive geological processes,” Cre- tional diamond layers on top of the ation, Vol. 16, No. 1, 1993, pp. 42– graphites would have converted them 45). GIA’s New York laboratory to covered internal features within recently received a 1.30 ct Fancy the larger diamond (R.H. Mitchell, brownish greenish yellow diamond Kimberlites and Lamproites: Pri- (figure 1) containing an octahedral- mary Sources of Diamond, Geo- shaped inclusion outlined by minute Figure 1. This 1.30 ct Fancy science Canada Reprint Series 6, Vol. crystal inclusions along the junctions brownish greenish yellow dia- of the crystal faces. mond contains an octahedral- Gemological examination at 60× shaped inclusion. magnification reveals that the octahe- Figure 2. Minute graphite inclu- dral-shaped inclusion is outlined by sions outline the octahedral crys- numerous irregular dark crystals (fig- clusions are graphite crystals with a tal faces in this diamond. Field of ure 2). Advanced gemological analysis Raman peak at 1590 cm–1 (figure 3), view 7.19 mm. with UV-Vis and FTIR spectroscopy which corresponds to the graphite G confirmed that this was a natural dia- band (I. Childres et al., “Raman spec- mond with a natural color origin. troscopy of graphene and related ma- Further analysis using Raman terials,” in J.I. Jang, Ed., New spectroscopy reveals that the dark in- Developments in Photon and Mate- rials Research, Nova Science Publica- tions, 2013). Since this G band is at a slightly higher energy level than that from the primary graphite, which Editors’ note: All items were written by staff usually peaks at around 1580 cm–1, members of GIA laboratories. we propose that the crystals tested GEMS & GEMOLOGY, Vol. 51, No. 4, pp. 428–440. are likely the secondary graphite con- © 2015 Gemological Institute of America verted from part of the original dia- mond into graphite form during the 428 LAB NOTES GEMS & GEMOLOGY WINTER 2015 RAMAN SPECTRUM diamonds. As seen in figure 6 (left), the cubic {100} sector can be located in the pavilion, along with octahedral 7000 {111} sectors. In order to understand more about this growth, we compared it with a type IIa 0.27 ct HPHT syn- thetic, which was treated post-growth to induce a Fancy Intense orangy pink 6000 color (figure 6, right). Both specimens showed similar fluorescence patterns; however, the natural diamond’s 1590 COUNTS greenish blue phosphorescence was 5000 evenly distributed, as seen in the mid- dle image of figure 6. This is unlike the dark images yielded from type IIa pink HPHT synthetics, which are not phosphorescent. 4000 Although most natural diamonds show octahedral growth structure in 2000 1900 1800 1700 1600 1500 1400 DiamondView images, some natural gem-quality colorless and yellow dia- RAMAN SHIFT (cm–1) monds may show cuboctahedral growth (Winter 2010 Lab Notes, pp. Figure 3. The graphite G band at 1590 cm–1 is found on the minute crys- 298–299; Winter 2011 Lab Notes, p. tals along the crystal junctions, confirming that they are graphite crystals. 310; Spring 2013 Lab Notes, pp. 45– 46). Some non-gem-quality diamonds possess cuboctahedral form (D.G. Pearson et al., “Orogenic ultramafic rocks of UHP (diamond facies) ori- 18, No. 1, 1991, pp. 1–16). Primary Treated Pink with HPHT Synthetic gin,” in R.G. Coleman and X. Wang, graphite crystals could also mix into Growth Structure Eds., Ultrahigh Pressure Metamor- the crystal clouds during the dia- HPHT synthetic diamonds are grown phism, Cambridge University Press, mond’s growth due to changes in en- using high pressure and high tempera- 1995, pp. 456–510). Growth rate, pres- vironmental temperature, pressure, ture but with a much higher growth sure, and temperature of geological and the growth fluid’s chemical ele- rate than natural diamond. Conse- environment control the habit of a di- ments. These minute graphite crys- quently, their growth structures are dif- tals would tend to form along the ferent. GIA’s New York lab recently junctions of crystal faces, since they examined a multi-step treated diamond usually have a higher surface energy. with a growth structure similar to that Figure 4. This multi-step treated Thus, we believe that both primary of HPHT synthetics. The 1.62 ct Fancy 1.62 ct Fancy pink natural dia- and secondary graphite formation, oc- pink type IIa round brilliant seen in fig- mond shows what appears to be curring between the diamond’s ure 4 showed spectral characteristics HPHT synthetic growth structure. growth episodes, contributed to this suggestive of HPHT treatment, irradi- phenomenal octahedral outline. ation, and annealing. This diamond Graphite inclusions are commonly was internally clean, except for the seen in diamonds as isolated crystals presence of strong, colorless internal or jointed crystal clouds. It is very un- graining (figure 5). Tatami and banded usual to see these minute graphite in- strains with strong interference colors clusions formed at the junctions of the could be seen under cross-polarized original diamond crystal faces and out- light (again, see figure 5). Based on these lining the octahedral growth pattern. microscopic features, we concluded This stone not only captures the that this was a natural diamond. amount of stress and extreme condi- DiamondView imaging of the tions under which the diamond grew, stone showed cuboctahedral growth but also shows the beauty of the for- with dark sectors and pink coloration mation of crystalline diamond. (figure 6). This crystal habit is usually Yixin (Jessie) Zhou observed in HPHT-grown synthetic LAB NOTES GEMS & GEMOLOGY WINTER 2015 429 Figure 5. Tatami strain (left, field of view 3.20 mm), banded strain (middle, field of view 3.20 mm), and heavy in- ternal graining (right, field of view 2.45 mm) are evidence of a natural diamond. amond crystal. A slow growth rate Very Large Type Ib Natural cently tested a large 13.09 ct gem- causes octahedral crystal growth, but Diamond quality natural type Ib rough dia- a high growth rate can facilitate cubic Nitrogen is the main impurity in dia- mond. The diamond measured 13.51 {100} and octahedral {111} sectors de- mond and during growth it is initially × 11.56 × 10.18 mm, with rounded do- veloping simultaneously. As a result, incorporated in the diamond lattice as decahedral morphology and clear dis- cuboctahedral habit can be formed isolated nitrogen atoms (C centers). solution pits on some faces in a naturally. Diamonds where the majority of nitro- symmetrical pattern (figure 7). The di- While the DiamondView is very re- gen occurs in C centers are known as amond had a highly saturated, slightly liable at revealing the growth structure type Ib. While C centers are common orangy yellow color evenly distrib- of natural vs. synthetic diamond, it is in lab-grown HPHT diamonds, the oc- uted throughout the whole crystal. important to correctly interpret all currence of C centers in natural cra- The diamond contained two tiny sul- identifying characteristics. When tonic diamonds is extremely rare (<1% fide inclusions with their associated cuboctahedral growth structure is ob- of all natural diamonds). Natural dia- graphitic rosette fracture systems. served, one should look for other monds form deep in Earth’s mantle, The infrared absorption spectrum features to support the origin. Micro- where high temperatures result in ni- revealed typical features of natural scopic features and spectral character- trogen aggregating to form A centers type Ib diamonds: a sharp peak at istics are useful for this purpose. –1 (N ) and B centers (N V). 1344 cm and a broad band at 1130 S 4 Kyaw Soe Moe and Wuyi Wang GIA’s New York laboratory re- cm–1. The absorption attributed to the Figure 6. DiamondView fluorescence images of the treated 1.62 ct pink natural (left) and the post-growth treated 0.27 ct pink HPHT synthetic (right) showed similar growth patterns, containing cubic {100} sector and octahedral {111} sectors. The middle image of phosphorescence of the 1.62 ct pink natural diamond showed even color distri- bution without growth sectors. Type IIa pink HPHT synthetic diamonds are usually not phosphorescent. {111} {111} {111} {100} {111} {100} {111} {111} 430 LAB NOTES GEMS & GEMOLOGY WINTER 2015 monds. Preservation of C centers in hyalite opal from Zacatecas, Mexico,” these natural diamonds is typically at- Journal of Gemmology, Vol. 34, 2015, tributed to very young formation ages pp. 490–508). or to storage of these diamonds at GIA’s Tokyo laboratory recently cooler temperatures in Earth’s mantle had an opportunity to examine sev- than most other diamonds. Ongoing eral faceted hyalites from Zacatecas, isotopic age dating of sulfide inclu- Mexico, along with several rough sions, combined with temperature hyalites from other locations, includ- constraints, will provide the opportu- ing Japan, Hungary, and Argentina. nity to evaluate how these enigmatic There were two different types of type Ib diamonds are preserved in nat- Japanese hyalites in the study sam- ural cratonic settings.