J.C. (Hanco) Zwaan

• PhD
• Researcher at Naturalis Biodiversity Center

Alluvial diamonds have been found in Suriname since the late 19th century, but to date the details of their origin remain unclear. Here we describe diamonds from Paramaka Creek (Nassau Mountains area) in the Marowijne greenstone belt, Guiana Shield, north-eastern Suriname. Thirteen samples were studied, consisting mainly of euhedral crystals with d...

Gem-quality rhodochrosite is rarely encountered, and is here described and characterised from the few known localities in the USA (Colorado), China, South Africa, Peru, Argentina and Brazil. The gemstones examined for this study were all relatively clean, with the main internal features consisting of partially healed fissures and two- and three-pha...

Although the source rocks of alluvial sapphires in Montana have never been discovered, inclusions and geochemistry of material from this location may give clues to their original source. Mineral inclusions in alluvial Montana sapphires, mainly from Rock Creek, were identified and compared with existing data. Topaz was a remarkable find in one of th...

Gem-quality cordierite is typically seen as blue material (iolite), showing strong pleochroism in yellow, light blue and dark violet-blue. However, in late 2014 one of the authors (FP) learned about a new occurrence of a much different cordierite, which typically ranged from dark orange-red to yellowish brown. The material is recovered by local min...

569 rich volcanic glass containing plagioclase and phyllosilicate inclusions, together with secondary calcite veins. This ornamental rock makes an attractive addition to the gem market, especially as it is relatively tough and hard, and thus shows a high lustre combined with an attractive colourful brecciated texture.

Seagrass meadows are important shallow-water ecosystems that provide habitat for numerous associated organisms and play a crucial role in nutrient cycling, but their detection in the fossil record is problematic. Indirect indicators are often needed to discriminate seagrass beds from other shallow marine paleohabitats. Here, the stable isotope sign...

Although the source rocks of alluvial sapphires in Montana have never been discovered, inclusions and geochemistry of material from this location may give clues to their original source. Mineral inclusions in alluvial Montana sapphires, mainly from Rock Creek, were identified and compared with existing data. Topaz was a remarkable find in one of th...

Recently, some natural pearls were found in Zeeland, The Netherlands, reportedly from the Grevelingenmeer and Westerschelde areas. Three whole pearls and one blister, still attached to its shell, were examined. These non-nacreous pearls were white, although the blister also had a light grey portion. They displayed a dull lustre and showed an uneven...

The Sleeping Lion is one of the largest-known nacreous pearls and has an interesting history. Newly discovered early references indicate that soon after the pearl was sold in Amsterdam in 1778, it was offered for sale in St. Petersburg in 1779. The pearl was recently removed from its setting, allowing its weight to finally be determined at 2,373 gr...

Zwaan, J.C., 2014. Gem Notes: Blue Kyanite from Tanzania. Journal of Gemmology, 34, 3: 198-200.

In 2006, emeralds were discovered in the Fazenda Bonfim region of northeastern Brazil. Emerald mineralization occurs in association with small recrystallized pegmatitic bodies hosted by metamorphosed ultramafic rocks within the Santa Monica Shear Zone. Prospecting and exploration have been carried out in a few small pits and tunnels, producing emer...

In 2006, emeralds were discovered in the Fazenda Bonfim region of northeastern Brazil. Emerald mineralization occurs in association with small recrystallized pegmatitic bodies hosted by metamorphosed ultramafic rocks within the Santa Monica Shear Zone. Prospecting and exploration have been carried out in a few small pits and tunnels, producing emer...

Copper-bearing tourmaline from Mozambique was first recovered in 2001, but its Cu content was not recognized until 2003, and it was not widely sold with its Mozambique origin disclosed until 2005. It has been mined from alluvial deposits in an approximately 3 km2 area near Mavuco in the eastern portion of the Alto Ligonha pegmatite district. Most o...

As one of the most valuable gemstones, emeralds are known to occur in several countries of the world, such as Colombia, Zambia, Brazil, Pakistan, Afghanistan, Russia, Madagascar and Zimbabwe. The emerald deposits at Sandawana, Zimbabwe, are described, the emeralds from this deposit characterised and a model of emerald formation presented; this is c...

Zambia is considered the world's second most important source of emeralds by value (after Colombia). The deposits are located near the Kafubu River in the Ndola Rural Restricted Area. Emeralds have been known from this region since 1928, but significant commercial production began only in the 1970s. As of mid-2004, most of the emeralds were being m...

In: Lindblom, J., 2005, Mineralogical Studies on Luminescence in Diamond, Quartz and Corundum. Turun Yliopiston Julkaisuja Annales Universitatis Turkuensis, Sarja - Ser. AII Vol. 183, pp. 1-26.

This study provides the first quantitative geochemical, petrological, and mineralogical data on major rock types and minerals in the Kafubu area. Highly magnesian talc-chlorite ± actinolite ± magnetite metabasites hosting emerald mineralization are identified as metamorphosed komatiites. These rocks contain 3000–4000 ppm Cr, and provided the amount...

Given the wide range of oxygen isotopic composition of emerald from all over the world (δ18O between +6.2 and +24.7‰), the δ18OV-SMOW values of emeralds from the Sandawana mines in Zimbabwe (δ18O‰=+6.6 to +8.0), are relatively constant, among the lowest ever measured. These consistently low values can be explained by host-rock buffering in a very n...

The National Natuurhistorisch Museum, Leiden (NNM), and the Teyler Museum, Haarlem, are contrasting museums separated more by their building style and history than the geographical distance. The NNM includes a collection of over two million minerals, rocks and fossils housed in a new, purpose-built exhibition building and associated 20-storey colle...

A large collection of emeralds of various occurrences has been analysed by PIXE/PIGE in view to establish a compositional database. Major elements (Be, Si, Al) and trace elements (Li, F, Na, Mg, Ca, Rb, Cs and transition metals) are determined using an external 3 MeV proton micro-beam. Elemental micro-mapping permits to select the useful provenance...

The Bamble sector of southern Norway is a classic high grade metamorphic gneiss region, which provided specimens to many mineralogical collections all over the world. The topographical mineralogy of this area is described and reviewed. All minerals known to occur in the area are listed according to Strunz’ classification.

Zimbabwe's Sandawana mines have been an important producer of emeralds for 40 years. Since the mines came under new ownership in 1993, consistent production has been established and, in addition to the small sizes for which Sandawana is known, greater numbers of polished stones as large as 1.50 ct have been produced. Currently, mining at the most a...

The Dr H.M.E. Schürmann collection is stored at the National Museum of Natural History in Leiden, The Netherlands. It mainly consists of Precambrian rocks, but also includes other crystalline rocks and minerals, which were collected from all over the world. In order to provide a quick reference to localities of interest, an overview of the most imp...