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Gemstones
Abstract
The sparkle and luster of gemstones has made them prized objects for thousands of years. Gems are valued for their color, luster, transparency, durability and high value-to-volume ratio. Diamond crystallizes in the cubic system, meaning that its constituent carbon atoms are arranged in cells with axes of the same length. Diamonds are divided into types according to the presence or absence of nitrogen and boron, as well as the structural organization of these impurities within the crystal lattice. The rest originate largely from sublithospheric sources, which can be as deep as the lower mantle. Such sources are generally described as being within deep keels of ancient cratons, where geothermal energy is suppressed by these relatively cold masses, thus allowing crystallization to occur. Well-formed diamond crystals most likely result from two processes. One is the reduction of oxidized carbonatein its solid state, or dissolved within a melted rock or chemical-rich fluid.
... Minerals 2019, 9, 461 2 of 29 ruby (red-colored corundum) and sapphire (blue-colored corundum), emerald and other gem forms of the mineral beryl, chrysoberyl, tanzanite (blue zoisite), tsavorite (green grossular garnet), "paraiba-type" tourmaline (copper-colored "neon"-blue tourmaline), topaz and jadeite-"jade" (jadeitite) [4]. More common gem materials include, among others, amber, silica gems, feldspar, tourmaline, spinel, garnets, zircon etc. ...
... According to Groat [3,4] and Stern et al. [5], any mineral or stone beautiful enough to be sought, mined, and sold for its beauty alone is a gemstone. Among the most important gemstones are diamond, ...
... Jadeitite, a rock that is also termed jadeite "jade", consists almost entirely of the pyroxene mineral jadeite. Its green color is attributed to iron substituting for aluminum in the jadeite [Na(Al,Fe 3+ )Si 2 O 6 ] crystal structure [4]. Jadeitite together with omphacitite occurs within the Kampos mélange, Syros Island in contact relationship or enveloping eclogite [27,44,45] as shown in Figure 5a,b. ...
In the Hellenides Orogen, minerals of various gem quality occur in various rock types from mainly four tectono-metamorphic units, the Rhodope, Pelagonian, and the Attico-Cycladic massifs, and the Phyllites-Quartzites unit of Crete Island. In crystalline rocks, gemstones are related to both regional metamorphic-metasomatic processes (e.g., gem corundums, Mn-andalusite, thulite/clinothulite, spessartine, titanite, jadeite), and to the formation of late alpine-type fissures, such as, for example, quartz, albite, adularia and titanite. The Tertiary (and Mesozoic) magmatic-hydrothermal environments provide gem-quality sapphire, beryl, garnet, vesuvianite, epidote, fluorite, and SiO2 varieties. The supergene oxidation zone of the Lavrion deposit hosts gem-quality smithsonite and azurite. Coloration in the studied material is either due to various chromophore trace elements present in the crystal structure, or due to inclusions of other mineral phases. Future modern exploration methods combined with gemological investigations (such as treatment and faceting of selected stones), are necessary in order to obtain a better knowledge of the gemstone potential of Greece for its possible exploitation.
... Apart from pearls, amber, coral and other organic materials, most gems in the world are single crystals of natural minerals, and a few are rocks, as well as a part of amorphous materials (such as Opal and natural glass) [1,2]. Moreover, in a broader sense, any mineral, stone or material that is beautiful enough to be sought, mined, and sold for its beauty alone can be used as a gemstone based upon Groat [3] and Stern et al. [4]. In detail, the most important gemstones are diamond, ruby, sapphire, chrysoberyl, emerald, jadeite, and nephrite. ...
Gemstones are minerals of gem qualities used for adornment and decoration with the attributes of beauty, durability and rarity. Traditionally, although China has been regarded as the most important source for nephrite, over the past decades, a large variety of gemstone resources have been newly discovered in China owing to continuous exploration works. The vast land with various geological and geochemical backgrounds is rich in gemstone resources with potential for new deposits discoveries. In pegmatites, gemstones are related to granitic magma events and mainly occur in pegmatitic cavities, such as tourmaline, aquamarine, spodumene, spessartine, moonstone, quartz, apatite, and topaz. The eruption of Tertiary basaltic magma provides gem-quality sapphire, spinel, olivine, garnet, and zircon. The supergene oxidation zones of some copper and iron deposits in Hubei and Anhui province host gem-quality turquoise and malachite. Moreover, the formation of the nephrite deposit in China is mostly related to the carbonatite and serpentinite rocks involved in the metamorphic-metasomatic processes. This paper comprehensively introduces the distribution of gemstones deposits, as well as the gemological and mineralogical characteristics of gemstones in China. Our present investigation provides insights into the gemstone potential of China for further exploitation.
The gemstones jadeite and ruby generally form as a result of the plate tectonic processes subduction and collision. Jade made of jadeite (jadeitite) forms when supercritical fluids released from subducting oceanic crust condense in the overlying mantle wedge, 20-120 km deep in the Earth. Jadeitite deposits thus mark the location of exhumed fossil subduction zones. Ruby, the red gem variety of corundum, forms during amphibolite- and granulite-facies metamorphism or melting of mixed Al-rich and Si-poor protoliths, 10-40 km deep in the crust. Suitable conditions generally exist where passive-margin carbonates and shales are involved in continental collision. Most ruby deposits formed during Ediacaran-Cambrian (ca. 550 Ma) collisions that produced the East African-Antarctic orogen and the supercontinent Gondwana, or during Cenozoic collisions in south Asia. Ruby is thus a robust indicator of continental collision. As a result of these diagnostic properties, we propose the term "plate tectonic gemstones" (PTGs) for jadeitite and ruby. The PTGs are a new type of petrotectonic indicator that are mostly found in Neoproterozoic and younger rocks. The PTGs as petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion, although the possibility of destruction via retrogression needs to be further assessed. Recognition of the PTGs links modern concepts of plate tectonics to economic gemstone deposits and ancient concepts of beauty, and may aid in exploration for new deposits.
The Taylor 2 (also known as Ghost Lake) emerald showing near Dryden, in northwestern Ontario, is associated with a granitic pegmatite of the Mavis Lake Pegmatite Group proximal to the 2685 Ma Ghost Lake Batholith. The Taylor 2 pegmatite consists of three separate limbs that intruded a wide zone of chlorite schist near the eastern end of an altered ultramafic sill. Most of the beryl and emerald occurs in a "zone of mixing" between the southern and central limbs of the pegmatite. The rock in the "zone of mixing" consists of relict orange K-feldspar crystals (to 30 cm) in a matrix of anhedral bluish plagioclase, quartz, fine-grained black phlogopite, blue apatite crystals (to 1 cm), and black tourmaline crystals (to 2 cm). The beryl occurs as euhedral crystals up to 2.3 X 1.8 cm; most are opaque to translucent and white to pale green in color, but about 10% are emerald. The emerald compositions show average Cr concentrations of 0.25 wt.% Cr 2O 3 (maximum 0.42 wt.% Cr 2O 3 or 0.03 Cr apfu) and maximum V concentrations of 0.05 wt.% V 2O 3. Primary fluid inclusions in the emerald crystals show variable compositions and high temperatures of homogenization (to 490°C). Oxygen isotope compositions of the emerald crystals, which average 9.6(7)‰, suggest a high degree of homogenization and fluid buffering during fluid-rock interaction. Whole-rock compositions show high concentrations of Be (89 ppm) in the the Taylor 2 pegmatite and elevated Cr in the chlorite schist (2610 ppm) and the altered ultramafic sill (3050 ppm). Geochemical similarities support the hypothesis that the chlorite schist is the faulted analogue of the altered ultramafic sill. The absence of beryl in the latter unit may be due to lower amounts of fluid or F concentrations (∼150 ppm versus ∼1300 ppm for the chlorite schist). The emerald in the Taylor 2 showing and the associated phlogopite most likely formed through metasomatism of the chlorite schist, which was driven by granitic magmatism.
Tourmalines from the Habachtal emerald deposit in the Eastern Alps formed together with emerald in a ductile shear zone during blackwall metasomatism between pelitic country rocks and a serpentinite body. Electron microprobe and secondary ion mass spectrometric (SIMS) analyses provide a record of chemical and B-isotope variations in tourmalines which represent an idealized profile from metapelites into the blackwall sequence of biotite and chlorite schists. Tourmaline is intermediate schorl-dravite in the country rock and become increasingly dravitic in the blackwall zones, while F and Cr contents increase and Al drops. Metasomatic tourmaline from blackwall zones is typically zoned optically and chemically, with rim compositions rich in Mg, Ti, Ca and F compared with the cores. The total range in δ11B values is −13.8 to −5.1‰ and the within-sample variations are typically 3–5‰. Both of these ranges are beyond the reach of closed-system fractionation at the estimated 500–550°C conditions of formation, and at least two boron components with contrasting isotopic composition are indicated. A key observation from tourmaline core analyses is a systematic shift in δ11B from the country rock (−14 to −10‰) to the inner blackwall zones (−9 to −5‰). We suggest that two separate fluids were channeled and partially mixed in the Habachtal shear zone during blackwall alteration and tourmaline-emerald mineralization. A regional metamorphic fluid carried isotopically light boron as observed in the metapelite country rocks. The other fluid is derived from the serpentinite association and has isotopically heavier boron typical for MORB or altered oceanic crust.
The different color exhibited by ruby and emerald is a fundamental but still unsolved question. According to recent EXAFS measurements, such a difference can hardly be explained on the basis of a different average distance between Cr3+ and the six oxygen ligands. The puzzling difference in color between the two gemstones is shown in this work to arise essentially from the distinct electrostatic potential imposed by the rest of lattice ions upon the active electrons of the CrO69− unit. Main effects are shown to come from the electric field generated in the neighborhood of the Cr3+ site in ruby which is absent in the case of emerald due to symmetry.
Oxygen isotopic compositions of historical emerald artifacts from the Gallo-Roman period to the 18th century indicate that
during historical times, artisans worked emeralds originating from deposits supposedly discovered in the 20th century. In
antiquity, Pakistani and Egyptian emeralds were traded by way of the Silk Route. Together with Austrian stones, they were
the only source of gem-quality emeralds. Immediately after the discovery of the Colombian mines by Spaniards in the 16th century,
a new trade route was established, first via Spain to Europe and India and then directly via the Philippines to India. Since
then, Colombian emeralds have dominated the emerald trade, and most of the high-quality emeralds cut in the 18th century in
India originated from Colombia.
The principal sources of natural diamonds are peridotitic (about 2/3 of diamonds) and eclogitic (1/3) domains located at 140-200 km depth in the subcratonic lithospheric mantle. There, diamonds probably form during redox reactions in the presence of melt (likely for eclogitic and lherzolitic diamonds) or under subsolidus conditions in the presence of CHO fluids (likely for harzburgitic diamonds). Co-variations of δ(13)C and the nitrogen content of diamonds suggest that two modes of formation may have been operational in peridotitic sources: (1) reduction of carbonates, that during closed system fractionation drives diamond compositions to higher δ(13)C values and lower nitrogen concentrations and (2) oxidation of methane, that in a closed system leads to a trend of decreasing δ(13)C with decreasing nitrogen. The present day redox state of subcratonic lithospheric mantle is generally too reduced to allow for methane oxidation to be a widespread process. Therefore, reduction of carbonate dissolved in melts and fluids is likely the dominant mode of diamond formation for the Phanerozoic (545 Ma-present) and Proterozoic (2.5 Ga-545 Ma). Model calculations indicate, however, that for predominantly Paleoarchean (3.6-3.2 Ga) to Mesoarchean (3.2-2.8 Ga) harzburgitic diamonds, methane reduction is the principal mode of precipitation. This suggests that the reduced present day character (oxygen fugacity below carbonate stability) of peridotitic diamond sources may be a secondary feature, possibly acquired during reducing Archean (>2.5 Ga) metasomatism. Recycling of biogenic carbonates back into the mantle through subduction only became an important process in the Paleoproterozoic (2.5-1.6 Ga) and diamonds forming during carbonate reduction, therefore, may predominantly be post-Archean in age. For eclogitic diamonds, open system fractionation processes involving separation of a CO(2) fluid appear to dominate, but in principal the same two modes of formation (methane oxidation, carbonate reduction) may have operated. Direct conversion of graphitized subducted organic matter is not considered to be an important process for the formation of eclogitic diamonds. The possible derivation of (12)C enriched carbon in eclogitic diamonds from remobilized former organic matter is, however, feasible in some cases and seems likely involved, for example, in the formation of sublithospheric eclogitic diamonds from the former Jagersfontein Mine (South Africa).
Tanzanite, a variety of the mineral found near Mount Kilimanjaro in Tanzania, has been growing in popularity as a precious stone. Tanzanite is a unique trichroic stone that radiates different colors in its rough form from each of its individual crystallographic axes including blue, violet, and burgundy. TanzaniteOne Limited, a Tanzania-based company, is focusing on developing a regulated market for the stone where supply matches and grows with demand. The worldwide market for rough tanzanite is estimated around US 150 million to US$200 million. Around 80% of the stone is cut and polished in India that is further exported to the USA, Europe, and the Far East. The company is also focusing on Hands Off Sorting, Grading and Recovery System (HOSGARS), which is expected to significantly improve security while enhancing the final grade of tanzanite recovered.
Emerald, the green gem variety of beryl, is the third most valuable gemstone (after diamond and ruby). Although it is difficult to obtain accurate statistics, Colombia supplies most (an estimated 60%, worth more than $500,000,000 per year) of the world's emeralds. However there is speculation that the emerald mines in Colombia are becoming depleted. Brazil currently accounts for approximately 10% of world emerald production. Emeralds have also been mined in Afghanistan, Australia, Austria, Bulgaria, China, India, Madagascar, Namibia, Nigeria, Pakistan, South Africa, Spain, Tanzania, the United States, and Zimbabwe.
Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified. Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons.
Dark blue aquamarine and beryl were discovered at the True Blue showing in the southern Yukon Territory in 2003. Electron-microprobe-derived compositions show up to 5.39 wt.% FeO in the darkest material, which is among the highest Fe concentration known for true beryl. The Al site totals average 2.05, with a maximum of 2.10 apfu, which implies that there is more Fe present in the sample than can be accommodated at the Al position. Charge-balance considerations and Mössbauer spectra show that the Fe is present as both Fe^(2+) and Fe^(3+). Optical absorption and Mössbauer spectra and the results of the X-ray and neutron single-crystal refinements suggest that there is very little Fe at the tetrahedral or channel sites. Previous investigators have proposed that the color of blue beryl is due to intervalence charge-transfer (IVCT) between Fe^(2+) and Fe^(3+) cations. The anisotropy of the optical absorption spectra suggest that if the mechanism responsible for the color in our samples is IVCT, the vector between the ions involved must be oriented approximately parallel to c. The only vectors that fulfill this condition and have a realistic length (2.300 Å) are 4d–Al and 6g–Be. Given the close proximity of the Si positions (closer than any anion sites), it is difficult to conceive of the substitution taking place at the interstitial 4d site. However, Fe could substitute at the interstitial 6g position, but likely only in very small amounts, because of the need to maintain local charge-balance. Unfortunately, there is no evidence of this in the Mössbauer spectra or in difference-Fourier maps of the X-ray- and neutron-diffraction data. For the former technique, it is likely that any doublet arising from Fe in the 6gO_6 polyhedron is too similar to the Fe in the AlO_6 octahedra to be resolved for either Fe^(2+) or Fe^(3+). Calculations suggest that the concentration of Fe involved in the IVCT process is 0.08 apfu Fe, of which half (0.04 apfu, 0.17 e^–) would potentially be at the interstitial site. This amount of electrons and this nuclear density are likely too small to be seen on the difference-Fourier maps.