Dr. J.C. (Hanco) Zwaan

Research interests

Gemmology of copper-bearing elbaite from Mozambique 

The focus of all studies is on gemstones. A current project is on copper-bearing elbaites from Mavuco, Nampula province, Mozambique. The geology and gemological properties of highly valued tourmalines from this new deposit are described. The characteristics of the material before and after heat-treatment are also studied. This project is carried out in cooperation with collegues at the Gemological Institute of America (Carlsbad, CA, and New York) and the University of New Orleans.

Large crystal fragments (pebbles) of copper-bearing tourmaline are found in the Nampula province of Mozambique. This example shows a natural 'neon blue' colour, equivalent to the colours that previously were solely known of copper-bearing tourmaline from the Paraìba state, Brazil. Photo by Hanco Zwaan © Naturalis.

In Spring 2009 it was announced that Zwaan and co-authors won the 'Dr. Edward J. Gübelin Most Valuable Article Award 2008 - first prize', as voted by the readers of the American journal Gems and Gemology, with the article 'Copper-bearing (Paraíba-type) Tourmaline from Mozambique'.

Emerald formation

A long-term research project is on the genesis of emerald in Precambrian greenstones. Processes related to the origin of emerald were studied at Sandawana, in Zimbabwe. A new mode of emerald formation is proposed, namely emerald formation as a product of contact metasomatism at the border of ultramafic rocks and rare-element pegmatites during a deformation event, involving late stage magmatic/hydrothermal activity channelled by shearing.  At Sandawana, this took place at the border of  a major Late Archaean suture zone. The model does not fit in any of the genetic classification schemes presented before and this study also underscores that no single theory or model can be applied to all schist-type deposits. 

Results of this case study are compared with other emerald occurrences, such as in Zambia and Brazil. General trends and/or differences will be highlighted.

Emerald crystal (length 4 cm, approx. 25 grams) from the Chantete mine, Kafubu area, Zambia. Like at many other emerald occurences around the world, emeralds found in Zimbabwe and Zambia were formed at the contact of ultramafic rocks and intruded rare-element pegmatites and hydrothermal veins. Photo by Hanco Zwaan © Naturalis.

In the framework of the Nederlands Edelsteen Laboratorium (Netherlands Gemmological Laboratory) the identification of gemstones and also pearls is part of the daily work. One of the research projects here is the use of micro-EDXRF applied to the systematic characterisation of ruby, sapphire, and emerald, from geologically different occurrences.

Netherlands Gemmological Laboratory

The Netherlands Gemmological Laboratory offers identification and grading services to Jewellers, Auction Houses, dealers, and consumers. Reports are issued on diamonds, coloured gemstones and pearls. Diamonds are graded according to international standards; the weight in carats, the colour, clarity and quality of the cut are evaluated. All these factors are important when the value has to be established of a particular stone. Because diamond is a valuable commodity, it is frequently simulated by other products, like synthetic cubic zirconia and synthetic moissanite.

Other gems, like ruby, sapphire and emerald, are frequently treated in order to enhance their appearance (colour and/or clarity), but also synthesized or simulated. So for each gemstone various categories exist, each representing a different value, for instance ‘natural ruby’, ‘enhanced ruby’, ‘treated ruby’, ‘synthetic ruby’ and imitation of ruby (often glass). In view of the difficulties in describing and identifying a gemstone correctly, the Netherlands Gemmological Laboratory is frequently asked for advice.

The Lab is independent and not involved in trading. It can therefore give an objective description and scientifically sound opinion on the quality and, where possible, provenance of any gem material that is offered for examination. The reports issued are internationally recognized and do not include valuation. In addition to relatively simple instruments used in earlier days, such as a refractometer, polariscope, and microscope, the laboratory (like other major gemmological laboratories in the world) now houses more advanced technology like Ultraviolet-Visible (UV-VIS) and Fourier Transform Infrared (FTIR) spectrometers, in order to measure absorption (or transmission) spectra of gemstones in the ultraviolet, visible and infrared frequencies, and energy dispersive X-Ray Fluorescence (micro-EDXRF) for non-destructive chemical analysis of gemstones. Additionally, radiography and X-Ray Diffraction techniques are used for pearl testing. 

Other activities include public lectures and various courses and seminars designed for professional jewellers and others, also in cooperation with the School for Gold- and Silversmiths and Jewellers (Schoonhoven). There is a strong link between the Lab, the collections, research, and public activities. The historical collections (e.g., gems donated by King William I) provide invaluable reference material for natural gemstones.

more information about the Nederlands Edelsteen Laboratorium (in Dutch)