A NEW CLASSIFICATION for RED-TO-VIOLET GARNETS by W

NOTES AND NEW TECHNIQUES

A NEW CLASSIFICATION FOR RED-TO-VIOLET GARNETS By W. William Hanneman

The recent article by Manson and Stockto17 on the shown in figure 1. The data points represent the classification of red-to-violet garnets should be ''trees." Now, let us loolz at the forest. welcomed by all gemologists. With this work, the The blaclz line connecting the pyrope and Gemological Institute of America hrrs opened the subject for review and revision. The following article almandine end .members defines the positions presents nn overall view of pl.evio~rsred-to-violet we would expect for all pure garnets comprising garnet classification schemes and proposes N new the pyrope-almandine solid-solution series. Points one that is well within the capabilities of every lying off the line result from either experimental gemologist. error or the presence of "other factors." Specifi- cally, if we disregard analytical error, points lying above the line suggest the presence of a substance having a positive effect on R.I. and a lesser posi- The major problem confronting gemologists with tive or negative effect on S.G. In this case, the respect to garnets has been that of "not being able cause may be chromium and calcium substi- to see the forest for the trees." The modem ten- tuting for aluminum and magnesium, respec- dency in gemology is to make more and more tively. Points lying below the line suggest the measurements on more and more specimens. This presence of something that effectively lowers the accomplishes the characterization of more and R.I. relative to the S.G. Such an effect could be ' more individual trees. Development of a useful produced by manganese substituting for iron or classification scheme for garnets requires that we by the presence of inclusions. stand baclz, loolz at the entire forest, and decide Because their statistical analysis indicated a what it is that we wish to accomplish with our high correlation for R.I., S.G., pyrope content, and classification. almandine content, Manson and Stoclzton divided In regard to the red-to-violet garnets, the data the red-to-violet garnet series into three groups: reported by Manson and Stoclzton (1981) repre- pyrope, pyrope-almandine, and almandine. The sent a prodigious effort. A plot of their data relating divisions between groups were arbitrarily set at refractive index (R.I.)to specific gravity (S.G.)is ratios of 60% pyrope:40% almandine and 60% almandine:40% pyrope, as determined by microprobe analyses. Data points in figure 1 are color ABOUT THE AUTHOR coded to reflect this scheme. The lack of a clear boundary for pyrope, as well as the presence of Dr. Hanneman is a research analytical chemist and founder of Hanneman Gemological Instruments, Caslro several serious divergences, indicated that this Valley, California. approach was not completely successful. "1983 Gemological Institute of America Be that as it may, Manson and Stoclzton devel-

Notes and New Techniques GEMS & GEMOLOGY Spring 1983 37 1.84 - 1.79 - Almandine I 1.83 - 1.78 - 1.82 -

1.81 - 1.77 - 1.80 - X ua, Pyrope- C Almandine x 1.79 - 1.76 w - u Y C -0 -w 1.78 - I .- 5 1.75 - I1I -s' 1.77 - 1.76 - 1.74 Pvrope 1.75 - 1.73 1.74 - 3.6 3.7 3.8 -3.9 4.0 4.1 4.2 Spec~ficGravity 1.73 -

1.72 - Figure 2. Graphical representation of refractive index and specific gravity limits for pyrope, Ill11 pyrope-almandine, and almandine garnets as 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 proposed by Manson and Stockton. The Specific Gravity lavender areas, although lying between the Anderson and Manson-Stockton lines, ore Figure 1. Data reported by Manson and undefined. Note overlap of areas. Stockton (1981) relating refractive index to specific gravity and color coded to indicate tively, they form the limits of the red-to-violet vntin nf nvvnno.nlmnnrlino nr Aoiorminorl hw 'UC'" V, yy'VyY.u'II'u"L.'"" UU L.wCw'A""'"U Yy garnet forest. The lower line, labeled Manson- microprobe analyses. Red indicates a ratio Stockton, can be considered a "base line." All of greater than 1.5:1, black indicates a ratio the garnets studied by these workers lie above between 1.5:1 and 1:1.5, and blue indicates a ratio less than 1:1.5. Lines labeled Anderson this line. The upper line, labeled Anderson, was and Manson-Stockton define an area within derived from B. W. Anderson's diagram of the which natural red-to-violet garnets have been pyrope-almandine series, which was reproduced described. Chrome pyrope values reported by by Webster (1975, p. 149). This line can be con- B. W. Anderson (1942) and Manson and sidered an expression of the upper limit of the red- Stockton (1981) ore designated by red squares. to-violet garnets, as determined experimentally by Anderson. The fact that it does not coincide oped what they stated to be a "more specific def- with the theoretical pyrope-almandine line indi- inition of the widely accepted terms pyrope, cates the influence of other factors in "real-world" almandine, and rhodolite for meaningful gemo- garnets. logical ~lassification.~'These authors proposed For nearly 50 years, the Anderson scheme has replacement of the term rhodolite as a garnet been in use practically everywhere except in group by the term pyrope-almandine, and set R.I. the United States. The scheme employs three and S.G. limits as illustrated in figure- 2. In order divisions-pyrope, pyrope-almandine, and alman- to evaluate this new definition, it is useful to dine-precisely those now advocated by Manson reexamine tneir1. aata (asI reproaucea* 3. in rigureC. 11., and Stoclzton. This author accepts without res- and relate it to previous publications of B. W. ervation the Manson-Stockton recommendation Anderson 11 947.. 19801 and R. Wehster 11 975) that the term rhodolite, as it has been used by U.S. gemologists, be replaced by the termpyrope- almandine. The question to be addressed here is COMPARISON OF THE B. W. ANDERSON whether the newly proposed Manson-Stoclzton AND MANSON-STOCKTON SCHEMES scheme presents sufficient advantages to displace Two red lines have been drawn in figure 1. Effec- the B. W. Anderson scheme.

38 Notes and New Techniques GEMS & GEMOLOGY Spring 1983 1.79 - 1.79 - Almandine Almandine (iron spectrum)

1.78 1.78 - r

1.77 1.77 - Xw u u Pyrope- c , AlmandinePyrope- C - 1.76 - 1.76 - Almandine w > .-- ,- - (iron spectrum) 0 L 0 m m r 51.75 5 1.75 a a -- Pyrope (iron spectrum) bnrome 1.74 /Pyrope 1.74 - Pyrope (dlromium r, spectrum) 1.73 , I ' 1 I II I I I 3.6 3.7 3.8 3.9 4.0 4.1 4.2 3.6 3.7 3.8 3.9 4.0 4.1 4.2 Spec~ficGrav~ly Spec~ficGravity

Figure 3. Graphical representation of refractive Figure 4. Graphical representation of proposed index and specific gravity limits for pyrope, scheme for the classification of the red-to-violet pyrope-almandine, and almandine garnets as garnets on the basis of refractive index and proposed by B. W.Anderson. The lavender visible spectrum. All colored areas lie between areas, although lying between the Anderson the Anderson and Manson-Stockton lines. and Manson-Stockton lines, are undefined.

It appears reasonably safe to postulate that pyrope-almandine ranges: i.e., R.I. = 1.751 to almost all red-to-violet garnets likely to be 1.752, S.G. = 3.81 to 3.86. These deficiencies are encountered by a gemologist will have R.1.-S.G. unacceptable in light of the fact that the divisions coordinates somewhere between the Anderson are purely arbitrary, and there is no fundamental and the Manson-Stockton lines. Therefore, the characteristic that dictates that there should be merits of any classification scheme ought to be any divisions at all. related to how well that scheme covers all the Now, if neither of these schemes can be judged possibilities. The R.I. and S.G. limits for the fully satisfactory, there is room for improvement. Anderson and Manson-Stockton schemes are In the following paragraphs an alternative scheme graphically illustrated in figures 2 and 3. The lav- is presented. ender areas represent "possible" R.1.-S.G. coordinates that are not covered by the schemes and BACKGROUND FOR PROPOSED are potential sources of problems for gemologists. GARNET CLASSIFICATION SCHEME It is evident that neither scheme covers the entire If one's concern is limited to only the red-to- range of possibilities. violet garnets, the assumption can be made that If it is accepted that the purpose of a classifi- the red color is due to the presence of iron andlor cation scheme is to enable the gemologist to make chromium. It is generally accepted that the iron a decision on the basis of his test results, the pres- is associated with the almandine end member and ence of an undefined area or an area of overlap is the chromium with the pyrope. Consequently, undesirable inasmuch as it makes any such deci- irrespective of their calcium or manganese con- sion impossible. The Anderson scheme (figure 3) tents, all red-to-violet garnets have traditionally could be improved by changing the arbitrary limits been considered members of the pyrope-alman- of S.G. from 3.95 to 4.00 for the pyrope-almandine dine series. mixture, and from 3.80 to 3.85 for pyrope. The The spectral characteristics of the red-to-violet Manson-Stockton scheme (figure 2) suffers from garnets are well described in the Manson-Stockton a gap in R.I. values between 1.774 and 1.779. In article. However, since pure pyrope garnet is col- addition, there is an overlap in the pyrope and orless, it is the iron (almandine)that produces the

Notes and New Techniques GEMS & GEMOLOGY Spring 1983 color and the spectrum that is the hallmark of the In the final analysis, this scheme is little more pyrope-almandine series. than the B. W. Anderson scheme with the S.G. Spectral evidence also indicates that pyrope limits removed. The problem of differentiating forms another series in which chromium substi- chrome pyrope from red spinel has been fully cov- tutes for aluminum and produces a red color. ered by Anderson (1980). Mineralogically spealzing, one might call this a The remaining problem is that of assigning a pyrope-lznorringite series. However, gemologi- classification to those low R.I. (<1.750) garnets cally speaking, this series does not appear to be that show spectral bands due to both chromium very extensive and this author proposes that its and iron. The criterion for malting this distinc- gem members be classified under the color variety tion is based on the results of Manson and chrome pyrope. This nomenclature is believed to Stoclzton and is as follows: If the three iron be gemologically consistent with varieties such absorption bands at 504, 523, and 571 nm can be as chrome tourmaline and chrome diopside. With observed with a hand spectroscope, the stone its intrinsic chromium content and unique should be classified as pyrope; chrome pyrope absorption spectrum, chrome pyrope deserves will exhibit absorption bands at 675 and 687 nm, recognition as a distinct variety and should not and will absorb virtually all light below 570 nm. be buried in the pyrope-almandine series. If sufficient chromium is present to mask at least Finally, the determination of physical con- two of the iron bands, the stone should be clas- stants should provide information on which to sified as a chrome pyrope. base a decision as to classification. Both R.I. and S.G. measurements reflect the same elements of SUMMARY composition. Anyone who has tried to identify Because of gaps and overlaps, the Manson- gems by S.G. measurements is well aware of the Stoclzton scheme for red-to-violet gamets cannot difficulty of obtaining accurate results as well as be considered acceptable. A different interpreta- the problems caused by inclusions. The R.I., how- tion of their data leads to the B. W. Anderson ever, is determined more easily and precisely. scheme, which is already in use outside the United Consequently, the scheme proposed here employs States. The S.G. limits of this scheme, however, R.I. determinations only, without S.G. are too narrow. Given the problems of deter- mining S.G. measurements, a simplified scheme PROPOSED RED-TO-VIOLET based on R.I. and spectral considerations has been GARNET CLASSIFICATION SCHEME presented. The "official" recognition of the color Acceptance of the preceding ideas leads to the variety chrome pyrope is recommended. conclusion that the red-to-violet garnets could be classified into two categories: pyrope-almandine EPILOGUE series and chrome-pyrope color variety. Insofar as If any revisions to garnet classification schemes neither the colorless pyrope nor the almandine are to be accomplished, it is imperative that input end members are known as gems, it would be rea- be received from many sources and a consensus sonable simply to call all the members of this reached. In an attempt to encourage a dialogue on series pyrope-almandine mixtures for the pur- this subject, D. V. Manson and C. M. Stoclzton, poses of a gemological classification. They are all whose work was discussed above, have been characterized by a typical almandine (iron)spec- invited to comment on this paper. Their response trum. Tradition, however, militates against this, follows. so the old divisions are retained. The proposed scheme is very simple. Chrome pyrope is characterized by its chromium spec- REFERENCES trum and by R.I. values below 1.750. Members of Anderson B. W. (1942) Gem Testing for Iewellers. Heywood the pyrope-almandine series are characterized by 8: Co., London. Anderson B.W. (1980) Gem Testing. Buttenvorth & Co., their iron spectrum. Pyrope exhibits an R.I. of London, pp. 328-329. 1.750 or less, while almandine exhibits an R.I. of Manson D.V., Stockton C.M. (1981) Gem gamets in the red- 1.780 or more. Specimens exhibiting interme- to-violet range. Gems d Gemology, Vol. 17, No. 4, pp. 191-204. diate R.I. values are designated pyrope-alman- Webster R. (1975) Gems. Buttenvorth & Co., London, pp. dine. The scheme is illustrated in figure 4. 145-152.

40 Notes and New Techniques GEMS & GEMOLOGY Spring 1983