Gem Localities of the 2000s

Shigley, James E.; Laurs, Brendan M.; Janse, A.J.A.; Elen, Sheryl; Dirlam, Dona M.

doi:10.5741/gems.46.3.188

Cited by 28 publications

(12 citation statements)

References 14 publications

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“…In the past 17 years, Madagascar has risen to become one of the world's largest producers of fine sapphires, ranging from the best blues to yellows, pinks, oranges and purples (Shigley et al 2010). Large investments by the World Bank to professionalise mining and to create sustainable livelihoods through, for example, the creation of lapidary training centres have had limited success.…”

Section: Lynda Lawsonmentioning

confidence: 99%

Rice, sapphires and cattle: Work lives of women artisanal and small-scale miners in Madagascar

Lawson¹

2018

Between the Plough and the Pick: Informal, Artisanal and Small-Scale Mining in the Contemporary World

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Section: Lynda Lawsonmentioning

confidence: 99%

Rice, sapphires and cattle: Work lives of women artisanal and small-scale miners in Madagascar

Lawson¹

2018

Between the Plough and the Pick: Informal, Artisanal and Small-Scale Mining in the Contemporary World

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“…There are several Be minerals that can be ranked among the gemstones such as euclase, phenakite and taaffeite, but only the true heavy mineral chrysoberyl and the modifications of transparent precious beryl (aquamarine/blue, morganite/pink, heliodor/yellow, emerald/green, goshenite/colorless, and pezzottaite/raspberry-red) are attractive for jewelers and mineral dealers [50][51][52][53][54][55][56][57][58][59]. The various varieties of precious beryl, although not a heavy mineral in the strict sense occur in alluvial (fluvial) placers and residual placers close to sites where the primary deposits are exposed to weathering and denudation (Figure 9a-c).…”

Section: Beryllium-bearing Gemstone Placers-beryl and Chrysoberylmentioning

confidence: 99%

Gems and Placers—A Genetic Relationship Par Excellence

Dill¹

2018

Minerals

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Gemstones form in metamorphic, magmatic, and sedimentary rocks. In sedimentary units, these minerals were emplaced by organic and inorganic chemical processes and also found in clastic deposits as a result of weathering, erosion, transport, and deposition leading to what is called the formation of placer deposits. Of the approximately 150 gemstones, roughly 40 can be recovered from placer deposits for a profit after having passed through the “natural processing plant” encompassing the aforementioned stages in an aquatic and aeolian regime. It is mainly the group of heavy minerals that plays the major part among the placer-type gemstones (almandine, apatite, (chrome) diopside, (chrome) tourmaline, chrysoberyl, demantoid, diamond, enstatite, hessonite, hiddenite, kornerupine, kunzite, kyanite, peridote, pyrope, rhodolite, spessartine, (chrome) titanite, spinel, ruby, sapphire, padparaja, tanzanite, zoisite, topaz, tsavorite, and zircon). Silica and beryl, both light minerals by definition (minerals with a density less than 2.8–2.9 g/cm3, minerals with a density greater than this are called heavy minerals, also sometimes abbreviated to “heavies”. This technical term has no connotation as to the presence or absence of heavy metals), can also appear in some placers and won for a profit (agate, amethyst, citrine, emerald, quartz, rose quartz, smoky quartz, morganite, and aquamarine, beryl). This is also true for the fossilized tree resin, which has a density similar to the light minerals. Going downhill from the source area to the basin means in effect separating the wheat from the chaff, showcase from the jeweler quality, because only the flawless and strongest contenders among the gemstones survive it all. On the other way round, gem minerals can also be used as pathfinder minerals for primary or secondary gemstone deposits of their own together with a series of other non-gemmy material that is genetically linked to these gemstones in magmatic and metamorphic gem deposits. All placer types known to be relevant for the accumulation of non-gemmy material are also found as trap-site of gemstones (residual, eluvial, colluvial, alluvial, deltaic, aeolian, and marine shelf deposits). Running water and wind can separate minerals according to their physical-chemical features, whereas glaciers can only transport minerals and rocks but do not sort and separate placer-type minerals. Nevertheless till (unconsolidated mineral matter transported by the ice without re-deposition of fluvio-glacial processes) exploration is a technique successfully used to delineate ore bodies of, for example, diamonds. The general parameters that matter during accumulation of gemstones in placers are their intrinsic value controlled by the size and hardness and the extrinsic factors controlling the evolution of the landscape through time such as weathering, erosion, and vertical movements and fertility of the hinterland as to the minerals targeted upon. Morphoclimatic processes take particular effect in the humid tropical and mid humid mid-latitude zones (chemical weathering) and in the periglacial/glacial and the high-altitude/mountain zones, where mechanical weathering and the paleogradients are high. Some tectono-geographic elements such as unconformities, hiatuses, and sequence boundaries (often with incised valley fills and karstic landforms) are also known as planar architectural elements in sequence stratigraphy and applied to marine and correlative continental environments where they play a significant role in forward modeling of gemstone accumulation. The present study on gems and gemstone placers is a reference example of fine-tuning the “Chessboard classification scheme of mineral deposits” [1] and a sedimentary supplement to the digital maps that form the core of the overview “Gemstones and geosciences in space and time” [2].

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“…Amethyst is a kind of α-quartz with beautiful violet color. It is widely distributed in the crust and the origins are all over the world, such as Brazil, Uruguay, Canada, Sri Lanka 1 , Rwanda 2 , Morocco 3 and Arizona 4 , etc. When the quartz contains impurities such as iron, aluminum and titanium, it will make quartz appear rich in color 5 .…”

Section: Introductionmentioning

confidence: 99%

Study on the effect of heat treatment on amethyst color and the cause of coloration

Cheng

Guo

2020

Sci Rep

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The effect of heat treatment on amethyst color was studied from a new perspective of chromaticity of gemstones and the cause of amethyst coloration was discussed based on the results of X-ray diffraction, ultraviolet–visible spectroscopy. The results show that the amethyst color has no significant relationship with cell parameters but the crystallinity index decreases as temperature rises. The absorption band at 545 nm in the UV–visible spectrum can be related to a charge-transfer transition of Fe3+ and O2−, which has a significant relationship with amethyst lightness and chroma. The color at different temperatures can be divided into three stages: The amethyst stage with temperature below 420 °C, the prasiolite stage with temperature between 420 and 440 °C where the color center is the most unstable, the citrine stage with temperature above 440 °C. The color change degree of heated amethyst is related to its initial color. When the initial color is darker, the color difference of heated amethyst is larger, and the easier it is to change the color after heat treatment. A more appropriate heating temperature to obtain citrine by heating amethyst is about 560 °C.

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Gem Localities of the 2000s

Cited by 28 publications

References 14 publications

Rice, sapphires and cattle: Work lives of women artisanal and small-scale miners in Madagascar

Rice, sapphires and cattle: Work lives of women artisanal and small-scale miners in Madagascar

Gems and Placers—A Genetic Relationship Par Excellence

Study on the effect of heat treatment on amethyst color and the cause of coloration

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