A description of pearl farming with Pinctada maxima in South East Asia

Professor H.A. Hänni. pp 357-365

This reports on a modern pearl farm in South East Asia which uses Pinctada maxima oysters. Marine biologists and geneticists supervise the pearling process. Culturing oysters from fertilized eggs has generally replaced wild oyster collection. In hatcheries larvae and spat are reared under scientifically controlled conditions. After around two years the oysters are about 12 cm in size and ready for the  operation. Careful selection of donor oysters for the tissue graft and of host oysters to grow the pearls  ensures optimum  onditions for pearl formation. Excellent environmental conditions are sought to grow  pearls of superior quality. High standards of working hygiene, X-ray checks and regular and frequent  leaning of the oysters and the holding nets are further steps to ensure high quality. Almost four years  rom the hatching of the larvae, the oysters are ready for a first pearl harvest. Most of the oysters are not re-seeded, and their muscle flesh is processed as seafood and the shells utilized for their nacre. The  earls are graded for quality and marketed mainly in Australia.

Gem-quality taaffeites and musgravites from Africa

Dr Karl Schmetzer, Dr Michael S. Krzemnicki, Prof. Dr Henry A. Hänni,Dr Heinz-Jürgen Bernhardt and Prof. Dr Thomas Pettke. pp 367-382

Gemmological, microscopic, chemical, and spectroscopic properties of a parcel of 30 taaffeites and ten musgravites of African origin, most probably all from Tunduru, Tanzania, are presented. The aceted gemstones were identified by a combination of laser Raman microspectroscopy, quantitative electron microprobe analyses and LA-ICP mass spectroscopy. The variation of gemmological roperties such as specific gravity and refractive indices is correlated with transition metal contents of the amples. Absorption bands in the UV-Vis range are assigned to iron in different valence states. Typical nclusions are primary apatite and magnesite crystals and healed feathers consisting of cavities that contain fluids and tiny secondary magnesite crystals. Gemmological and microscopic properties of these African stones are compared with characteristic features of taaffeites and musgravites from Sri Lanka. Most gem-quality taaffeites and musgravites from Sri Lanka and Africa originate from agnesian skarns or from alc-silicate rocks of metasomatic origin.

Some observations on the composition and origin of opals from Java

H.C. Einfalt. pp 383-398

The precious opal from Rangkasbitung, Banten Province in West Java occurs in a decomposed tuff layer as a component of the paragenesis montmorillonite – zeolite (clinoptilolite) – opal which formed in an open fresh water system at low temperatures. White, water, brown and black precious opals and non-precious opals from this area have been investigated by various methods. The gemmological characteristics are described. Internal features consist of flow texture, microcrystalline granular quartz/chalcedony, and inclusions of zeolites. Several opals show indications of stress (micro-fractures,  lide planes, deformed fields of play-of-colour) which is discussed as a possible reason for the poor stability of some Indonesian opals during processing and wear. Though all opals are optically isotropic, X-ray diffraction data indicate that three structural types are present: opal-A, opal-CT and a structurally intermediate opal-‘C’. All opals are composed basically of small granules (nanograins) of 30 to 50 nm diameter. Spherical, regularly arranged aggregates of these granules are visible only in opal-A  samples, whereas opal-CT and opal-‘C’ types appear relatively non-structured. Hydrofluoric acid-etched samples render visible in a SEM a very regular pattern of spheres of about 290 nm diameter in opal-A, but similar treatment of opal-‘C’ showed complete dissolution of spheres leaving a framework of pore cement. HF treatment of opal-CT left a massive cement and a hardly visible regular arrangement of only a few non-spherical voids. Analyses of the main elements in five representative opals indicate a wide range of concentrations for Al (4200-10100 ppm), Ti (3-5700 ppm), Ca (1480-6370 ppm), Na (700-2450 ppm) and K (390-1630 ppm) as the main impurities. Iron (6-405 ppm) is relatively low and is unlikely as the sole cause of dark brown and black body colours. Major trace elements of tens of ppm are Ba, Zr, Y and Rb. With the exception of unusual high values of Ti, Ca and Mg in one opal and the low Ba content, the chemistry does not provide a direct genetic link to the tuffaceous host rock..

Vaterite in freshwater cultured pearls from China and Japan

U. Wehrmeister, D. E. Jacob, A. L. Soldati, T. Häger and W. Hofmeister. pp 399-412

Japanese and Chinese tissue- and bead-nucleated cultured freshwater pearls of good quality were investigated with Raman spectroscopy and LA-ICP-MS. In 50% of the investigated tissue-nucleated samples vaterite was identified by Raman spectrometry in polished cross-sections near the centre of the pearl as well as in small blemishes on the pearl surfaces. Continuous growth structures transect both vaterite and aragonite areas. Sodium and strontium concentrations are significantly lower in vaterite areas whereas the Mg concentration is up to two orders of magnitude higher and ratios of these elements allow distinction of the two phases using LA-ICP-MS. With the findings that vaterite is relatively common in the centres of high-quality freshwater cultured pearls and is the reason for lack of orient on some pearl surfaces, it is necessary to fully understand how it forms in order to optimize the quality of such pearls.

Vanadium-bearing gem-quality tourmalines from Madagascar

Dr Karl Schmetzer, Dr Heinz-Jürgen Bernhardt, Christian Dunaigre and Dr Michael S. Krzemnicki. pp 413-433

Gemmological, microscopic, chemical, and spectroscopic properties of green, vanadium-bearing gem-quality tourmalines from Southern Madagascar are presented. The samples are iron- and lithium-free and are designated as calcic aluminous dravites. They reveal a small compositional variability for sodium, calcium, magnesium and aluminium. Positive correlations between Na and Al and between Ca and Mg are present, always with Al >6 atoms per formula unit and Mg <3 atoms per formula unit. Vanadium is the main transition metal present with smaller amounts of chromium and these cause the green coloration. Absorption bands in the UV-Vis range are assigned to V3+ on octahedral aluminium sites. Mineral inclusions in the tourmalines were characterized by laser Raman microspectroscopy and quantitative microprobe analyses. The commonest are bytownite plagioclases, but quartzes and zircons are also present as well as cavities and healed fractures containing liquid and two-phase fillings (liquid/gas). Non-homogeneous irregularly shaped grains consist of a mixture of hydrous aluminium silicates and iron hydroxides. The tourmalines from Madagascar are compared with iron-free or almost iron-free gem-quality tourmalines, mainly from East Africa. Correlation diagrams of Na, Ca, Mg, and Al show two different population fields, a) with Al>6 and Mg <3 atoms per formula unit (aluminous dravites) and b) with Al<6 and Mg >3 atoms per formula unit (uvites). The main isomorphic replacement within this solid solution series occurs between the tourmaline end-members oxy-dravite, Na(Mg₂Al)Al₆(BO₃)₃Si₆O₁₈(OH)₃O, and uvite, CaMg₃(MgAl₅)(BO₃)₃Si₆O₁₈(OH)₄, and is represented by the substitutional scheme Na⁺ + 2Al³⁺ + O^2- ↔ Ca²⁺ + 2Mg²⁺ + (OH)^- .

Clarification of measurement of the RIs of biaxial gemstones on the refractometer

B. Darko Sturman. pp 434-442

All possible refractometer observations on biaxial gemstones can be represented by four patterns shown in diagrams where RIs are plotted on the vertical axis and rotation angles on the horizontal axis. New understanding of the behaviour of biaxial gemstones on the refractometer is based on calculated movements of shadow edges for many different orientations of the optical elements and the gem table or a gem facet.

Determination of the optic axial angle in biaxial gemstones and its use in gemmology

B. Darko Sturman. pp 443-452

From the very beginning of the use of the refractometer in identification of gemstones, gemmologists faced two problems:
• how to differentiate between biaxial gemstones with similar or overlapping RIs
• how to distinguish a uniaxial and a biaxial stone in an orientation which produced one variable and one constant shadow edge.
The use of the polarizing filter to solve these problems can be avoided in many cases by the use of the optic axial angle method. The most important feature of this method is that it is reliable. Also, whether or not the method can be applied is quick to determine. Determination of an optic axial angle can be made on a simple diagram and the list of biaxial gemstones and their optic axial angles is shown alongside. Determination of the optic axial angle requires the same data as determination of the optic sign but it is a much more discriminatory constant for use in identification of biaxial gemstones.