This content was first published on IGR – Italian Gemological Review no. 15 in 2022. The information provided here is therefore current as of the original publication date.
The “IV National Conference Diamond and Coloured Gems: Identification, Tracing and Ethic Aspects” was held in Ferrara, Italy, on July 7-8, 2022 at Palazzo Tassoni Estense. As was the case for last edition in 2021 held in Bari, IGR promoted the event, which was well attended, contributing as Media Partner to its dissemination, at both national and international level.
A series of presentations focused on characterizations. Giada Musilli (University of Milan La Statale) presented the results of a research on Ethiopian emeralds, 11 specimens provided by Cisgem, studied through the use of FTIR, EDXRF, UV-Vis, micro-Raman and optical microscopy techniques. Ethiopian emeralds were found in 2004 in the Dubuluk area, but it was not until 2016 that gem quality specimens were mined in the Shakiso area. In 2017 they accounted for 1.5 percent of world supply. They are classified as “schist type IA”, derive from metasomatic hydrothermal processes, and their formation can be placed from Cambrian to Post Cambrian age.
In the literature they are commonly considered similar to those from Brazil and Zambia. Actually, differences from the former are found in the UV-Vis spectrum of the ε-ray and in FTIR spectra, with positional variations of peaks of H2O molecules of type I-II and CO2. They differ from the latter by higher Ga content and lower Mg/(Na+K). The discovery of clinochlore needle-like inclusions, observed only in Colombian emeralds, but always in a different habit, and dark needle-like inclusions terminating on one side with a hexagonal base, never highlighted until now, is an important indication. Characterization of these, together with isotopic investigations, would be very helpful in refining the discrimination between Ethiopian and emeralds from other provenances.
Francesco Maria Lo Forte (University of Palermo) has analyzed simetite amber samples selected from the mineralogical collection of the University of Palermo, determining that the color of simetite (yellow, orange, light red, dark red and black) turns darker as the amount of sulfur increases and oxygen decreases.
Sara Monico (University of Milan la Statale) presented the results she achieved in the study of aquaprase, a gemological variety of chalcedony showing a typical blue-green coloration, discovered in 2013 – in an undisclosed African locality – by Yanni Melas. The investigation was carried out employing XRPD (X-Ray Powder Diffraction), micro Raman, optical microscopy, EMPA (Electron Micro Probe) and LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry) and was based on a methodology that compared aquaprase with two widely documented chalcedony varieties, chrysoprase and blue agate.
Chalcedony, a microcrystalline variety of quartz, has an oriented fibrous texture, occurs with two polymorphs of silica and is a polychromatic aggregate. Mineralogical and spectroscopic investigations have shown that aquaprase has alpha quartz and morganite as its crystalline phases. The data confirm typical characteristics of chalcedony along with more specific ones: aquaprase has lower specific gravity and inhomogeneous coloration. The colorless areas are due to the presence of microcrystalline quartz, macrocrystalline quartz, and fibrous quartz.
The colored areas are explained by the presence of microcrystalline quartz associated with cryptocrystalline and fibrous quartz. Analysis of the chemical elements reveals that aluminum, Ni2, magnesium, and chromium (up to 0.3%wt) occur in higher percentages and are chromophore elements, while for other comparable varieties of chrysoprase the nickel value is higher.
The main features of crystal specimens of corundum variety ruby from the Prilep area (North Macedonia) were reported by Nicola Precisvalle (University of Ferrara). These rubies are known in the literature since 1961 though they were introduced on the jewelry market only since 1988, when they first appeared at the Tržic mineral fair in Slovenia. Commonly referred to as the “Macedonian ruby”, this stone emerges as the national gemstone of North Macedonia, so much so that it has become a common gift used in diplomacy. Rubies can be found in Precambrian marbles matamorphosed from granite action, in mines exploited from antiquity in the municipality of Prilep. The specimens usually occur with inclusions of oxides, carbonates (e.g., dolomite and calcite) and aluminum hydroxide. Their color range from pink, somewhat common among rubies, to a pleasant raspberry hue (light, moderately intense red, slightly purplish) and are more suitable for cabochon cutting, being they mostly highly included. These rubies are essentially characterized by the presence of typical diaspore inclusions that result in an unprecedented phenomenon, that is diasporescence, a milky effect with a certain iridescence due to the intercalation of diaspore crossing with corundum. An additional characteristic is its barrel-shaped growth, which makes the rough material attractive for mineral collectors and museums.
Article by Paolo Minieri and Stefania Coppola, published on IGR – Italian Gemological Review #15, Autumn 2022.
