corundum, ruby, sapphire, treatments.6, inclusions.5, Colombia.12, Brazil.12, USA.12, Montana.12, Idaho.12, gems

gems, beryl, emerald, Colombia

A condensed historical and mining survey is followed by a compilation of the most notable crystals of Colombian emeralds ever found and of the details of their chemical composition. A summary of the elemental substitutions and their effects on the outlined range of gemmological data is given. Special emphasis is placed on the cause of the famous, vividly green coloration, i.e. on the selective absorption of orange-red and violet light, dictated by minor contents of the chromophoric elements Cr and V substituting for Al³⁺ in octahedrally coordinated sites of the beryl lattice (Cr ≈ V, with the Cr/V ratio found to be ranging from 4.5 to 0.5, in extreme cases from about 10 to 0.1). Iron, however, has barely been detected in the UV/VIS absorption spectra of Colombian emeralds. Though generally present in slightly larger amounts than Cr³⁺, iron does not recognizably influence the colour (little Fe²⁺ and possibly some Fe³⁺ positioned in the axial channels of the ring-silicate structure). A detailed presentation of the currently known internal characteristics (inclusions and growth structures) is supplemented by a discussion of identification problems with treated emeralds and the disclosure of the treatment during transactions in the wholesale and retail trade. Allocation of the emeralds to individual Colombian mines on the basis of inclusion patterns does not appear possible in most instances. However, the differentiation of Colombian emeralds from other natural emeralds (such as those from the Panjsher Valley exhibiting very similar (s,l,g) three-phase inclusions) or from difficult, modern hydrothermal and flux synthetics is shown to be feasible by a combination of microscopy, refractometry, and absorption spectrometry (UV/VIS and NIR/MIR), assisted by chemical analysis only in the most difficult cases. Safe identification of authenticity, treatment, and origin of emeralds, however, is increasingly becoming the task of the experts in the specialized laboratory. The mineralogical definition of emerald as a yellowish to bluish-green variety of natural beryl is shown to be valid for any Cr and V-containing variety except possibly the rare, absolutely chromium-free (V,Fe)-beryls (modification of the former definition of type II emerald): in many of the preferred Colombian emeralds, chromium is clearly dominated by vanadium (Cr/V ratio < 1). However, chromium has a higher efficiency of coloration than vanadium and a much higher one than iron ions in the various substitutional and interstitial lattice sites of beryl. Cr influences colour even at very low trace levels. An extensive and up-dated selection of literature, covering all aspects cited, completes the synopsis.

gems, beryl, emerald, Colombia

A condensed historical and mining survey is followed by a compilation of the most notable crystals of Colombian emeralds ever found and of the details of their chemical composition. A summary of the elemental substitutions and their effects on the outlined range of gemmological data is given. Special emphasis is placed on the cause of the famous, vividly green coloration, i.e. on the selective absorption of orange-red and violet light, dictated by minor contents of the chromophoric elements Cr and V substituting for Al³⁺ in octahedrally coordinated sites of the beryl lattice (Cr ≈ V, with the Cr/V ratio found to be ranging from 4.5 to 0.5, in extreme cases from about 10 to 0.1). Iron, however, has barely been detected in the UV/VIS absorption spectra of Colombian emeralds. Though generally present in slightly larger amounts than Cr³⁺, iron does not recognizably influence the colour (little Fe²⁺ and possibly some Fe³⁺ positioned in the axial channels of the ring-silicate structure). A detailed presentation of the currently known internal characteristics (inclusions and growth structures) is supplemented by a discussion of identification problems with treated emeralds and the disclosure of the treatment during transactions in the wholesale and retail trade. Allocation of the emeralds to individual Colombian mines on the basis of inclusion patterns does not appear possible in most instances. However, the differentiation of Colombian emeralds from other natural emeralds (such as those from the Panjsher Valley exhibiting very similar (s,l,g) three-phase inclusions) or from difficult, modern hydrothermal and flux synthetics is shown to be feasible by a combination of microscopy, refractometry, and absorption spectrometry (UV/VIS and NIR/MIR), assisted by chemical analysis only in the most difficult cases. Safe identification of authenticity, treatment, and origin of emeralds, however, is increasingly becoming the task of the experts in the specialized laboratory. The mineralogical definition of emerald as a yellowish to bluish-green variety of natural beryl is shown to be valid for any Cr and V-containing variety except possibly the rare, absolutely chromium-free (V,Fe)-beryls (modification of the former definition of type II emerald): in many of the preferred Colombian emeralds, chromium is clearly dominated by vanadium (Cr/V ratio < 1). However, chromium has a higher efficiency of coloration than vanadium and a much higher one than iron ions in the various substitutional and interstitial lattice sites of beryl. Cr influences colour even at very low trace levels. An extensive and up-dated selection of literature, covering all aspects cited, completes the synopsis.

beryl, chrysoberyl, beryl, emerald, alexandrite, Russia, Urals, Malysheva, gems

Emeralds are again being mined in the historic district of the Ural Mountains. Physical, chemical, spectroscopic, and ini-croscopic characteristics of stones from the recent production are presented and com-pared with data on older samples as well as those reported in the literature. Chemi-cal analysis and visible and infrared spec-troscopy provide information on crystal chemistry and color-causing trace ele-ments that is useful both to characterize these stones and to separate them from their synthetic counterparts. A variety of mineral inclusions as well as liquid, two-phase, and three-phase inclusions were seen with magnification. All of the data are critically evaluated with respect to the confirmation of unknown samples as Uralian.

corundum, ruby, Kenya.12, East Africa, inclusions.5, gems

corundum, ruby, Tanzania.12, East Africa, inclusions.5, gems

The authors present the mineralogical and gemological properties of rubies found recently in the Morogoro area of Tanzania that have many features in common with rubies from Myanmar (Burma) and other comparable marble-type deposits. Specifically, microscopic investigations of these Morogoro stones revealed some internal characteristics that are also seen in Burmese rubies-such as angular growth zoning, curved growth zoning (swirls"), negative crystals, spinel crystals, rutile needles, and clouds of minute particles in areas defined by both types of growth zones, which may also be related to color zoning. However, the Morogoro material appears to have (1) lower amounts of V and Ga than Burmese rubies, and (2) a characteristic angular growth with r, a, and r' faces. A gemological comparison of the recently produced Morogoro rubies with rubies found earlier in this area shows significant differences."

gems, inclusions, beryl, emerald, Brazil

corundum, ruby, USSR.12, Tajikistan.12, inclusions.5, gems

Describes gem-quality rubies from a marble-hosted deposit in the Pamir Mountains of the then USSR, near the Chinese border. The stones range from red through reddish to violet-red and have SG 3.98, refractive indices nₑ 1.761–1.762 and nₒ 1.769–1.770, and birefringence 0.008. Microscopic features include twinning lamellae, healed fissures, fluid inclusions, rutile dust and distinct growth striae. Trace-element analysis detected chromium, iron and titanium; absorption spectra show the characteristic chromium bands, and the stones exhibit strong red fluorescence.

corundum, sapphire, Kashmir.12, India.12, gems

Describes an unusual engraved Kashmir sapphire weighing 6.175 ct and measuring 12.19 × 10.04 × 5.32 mm. The antique-cut stone has 32 crown facets, a thin girdle and a pavilion arranged in four groups of 16 facets. Its characteristic silk and microscopic inclusions establish its Kashmir origin. The engraving depicts a lion seated upon a crown and was produced with fine diamond points using non-rotating tools. The complexity of the design would have made the seal difficult to counterfeit, and evidence indicates that the sapphire was repeatedly used as an official seal. Diehl and Meng suggest that it may have belonged to an Indian ruling family, possibly Sikh, although neither its precise provenance nor its age could be established.

corundum, inclusions.5, India.12, Kashmir.12, gems