SPECTROSCOPIC AND CHEMICAL CHARACTERIZATION OF EMERALDS

SPECTROSCOPIC AND CHEMICAL CHARACTERIZATION OF EMERALDS

¹Aurisicchio C., ²Nunziante-Cesaro S., ¹Corami A.

¹IGG c/o Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza”,
P.le A.Moro 5, 00185 Roma, Italy. Carlo.aurisicchio@uniroma1.it;
²ISMN c/o Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”,
P.le A.Moro 5, 00185 Roma, Italy. s.nunziante@rm.cnr.it

XRD analyses show that a reticular parameter increases but c is quite constant, according to c versus a correlation. This confirms the occurrence of Al substitutions in the octahedral site. Studied samples are grouped in three areas related to their origin.

Spectroscopic studies were carried out both on solids and, when possible, on powdered samples. In the former case polarized spectra were also obtained. In the latter case, diffuse reflectance spectra were taken on weighted amounts of crystals dispersed in KBr excess. The same mixtures, pressed in pellets, were also analysed at room and low temperature (12K).

The comparison of spectra obtained under different experimental conditions revealed that skeletal modes, lying in the range 1300–400 cm^–1, are nearly unaffected both by the physical status and by the experimental conditions. However plots of the silicon-oxygen stretching modes at 1200 and 950 cm^–1 versus substitution in the octahedral coordinated site and alkali concentration show a good negative correlation.

Fundamental vibrations of H₂O molecules appeared much more intense in spectra of powdered than of solid samples. In the water stretching modes range (3700–3500 cm^–1) three distinct bands were normally observed showing negligible shifts in frequency but significant variations in their relative intensities. The sodium content is the main responsible of the intensity ratio of the bands observed which, as a consequence, are attributed to the presence of free water molecule and water-sodium complexes having both 2:1 and 1:1 ratio. Low temperature spectra of a number of samples show a marked increase of the intensity and splitting of the bands assigned to water-sodium complexes probably due to the stabilization of the complex itself and/or to its different orientation with respect to the axis of the channel.

The assignment of H₂O bending modes, in the 1700–1550 cm^–1 range, is grounded on their behavior versus the sodium content and the lowering of temperature strictly paralleling that observed for stretching fundamentals.

The determination of the total water quantity obtained by gas chromatography, and the knowledge of alkali content allow to estimate relative amounts of free and complexed water.

The present contribution is part of a wider work aiming to the identification of chemical-physical parameters useful to individuate archaeological gems origin. To this end, statistical analyses of the experimental results have been also employed, in the hypothesis to differentiate mines. The promising results are however affected by the limited crystallization environment of emeralds.

4C's and other characteristics of diamonds certified
by gem testing laboratory
of Spanish Gemmological Institute. Statistic study of 5200 samples

Gavrilenko E.V., Cozar J.S.

Instituto Gemologico Espanol, Madrid, Spain, http://www.ige.org, info@ige.org

M

Fig. 1. Weght distribution of analysed diamonds

ost important diamond characteristics commonly known as 4C's (Carat weight, Colour and Clarity grades, Cut parameters) were statistically studied for the last 5200 diamonds certified by IGE gem testing laboratory. Other characteristics such as UVL (365 nm) fluorescence and absorption spectra characteristics (the intensity of the 415.5 nm line) were taken into account. The relation between the UVL fluorescence and optic absorption was studied for colour D diamonds.

Carat weight. Weight distribution of analysed diamonds is very irregular and shows well defined maximums at nearly round weights (for example, 1.0–1.1, 0.5–0.6 weight groups), related with the diamond price changes at these points (Fig. 1).

Colour. Distribution of analysed diamonds by their colour grade, obtained as a result of comparison with diamond colour masterstones is shown in Fig. 2. In 101 diamond (1.9 % of all diamonds) rare non-compatible or fancy colours were observed.

C

Fig. 2. Distribution of analysed diamonds by colour grades

larity. Most of analysed diamonds has the VS2 clarity grade (Fig. 3). No P3 diamonds were observed in the last 5200 certified stones.

Cut. The majority (90.26%) of all diamonds have round brilliant cut. The distribution of their proportions values is shown in the Figures 4–9. Ideal values for two different round brilliant cut models (Tolkowsky and Tillander) are also plotted for comparison with the data given by real diamonds.

Other cut and finish parameters present the following distributions (% of all studied diamonds).

Cut type: brilliant — 93.04, emerald — 3.91, princess — 1.57, “old cut” — 1.08, others — 0.41.

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Fig.3. Distribution of analysed diamonds by clarity grades

ut shape: round — 91.76, rectangular — 4.46, square — 1.1, pear — 0.85, oval — 0.58, marquise — 0.58, hart — 0.39, triangle — 0.14, others — 0.14.

Girdle type: rough — 57.93, faceted — 37.81, partly polished — 0.41, partly faceted — 0.17.

Culet type: pointed — 82.65, polished — 9.47, slightly damaged — 7.79, natural — 0.09.

Symmetry: very good — 10.85, good — 82.43, medium — 5.67, deficient — 1.04.

Polish: very good — 27.23, good — 70.20, medium — 1.68, deficient — 0.89.

T

Figs.4–9. Cut parameters of round brilliant cut diamonds

S

Fig. 10. Distribution of colour D diamonds by values of blue UVL fluorescence and 415.5 nm absorption line intensity

tudy of the properties of colour D diamonds.

The study of 85 colour D diamonds (1.63 % of all analysed stones) were carried out to determine the correlation between their blue fluorescence and optic absorption spectra. In the Figure 10a one can observe that colour D diamonds tend to plot 2 families of values. First one characterises by the absence of absorption at 415 nm and very weak to medium fluorescence (dark columns, 42 stones, 53 % of all colour D diamonds). These are the diamonds which do not below to the Cape type.

Other group of colour D stones (47 %) is formed by Cape type diamonds generally with well pronounced absorption at 415.5 nm and strong ore medium fluorescence. Figures 10b y 10c represent the distribution of colour grades for all diamonds that have characteristics of these two groups

Conclusions

5. 
   The distribution of weight of analysed diamonds is very heterogeneous. The majority of diamonds belongs to the 1–1.1 or 0.5–0.6 ct weight groups.
   
6. 
   The most frequent grades of colour and purity of certified diamonds are H and VS2.
   
7. 
   90% of all diamonds have round brilliant cut. Other most used cuts are: emerald (3.91%) and princess (1.57%).
   
8. 
   The real values of pavilion depth and table width of round brilliant cut are usually bigger than ideal ones, while the crown height is usually smaller.
   
9. 
   Colour D diamonds represent 1.63% of all diamonds analysed. This colour grade can have both Cape type diamonds and diamonds which do not below to Cape type.
   
10. 
    Two groups of diamonds were find to have a great probability for high colour grades:
    

• 
  diamonds which have no absorption line at 415.5 nm;
  
• 
  those which have quite strong absorption at 415.5 nm but also medium UVL blue fluorescence.