Generalities: Greek ἀδάμας (adámas) = unbreakable (Pliny, 77 A.D.) → Metastable (!) allotrope of carbon (at standard conditions, graphite is stable; the transformation of diamond to graphite is however infinitely slow at standard conditions) Extraordinary properties among all minerals known, including: - highest hardness (Mohs 10) - highest thermal conductivity - highest melting temperature (∼3550 °C)

Gemology: Diamond

Structure and crystals: Crystal structure: - cubic (isometric-hexoctahedral), Fd3m - a0 = 3.5595 Å; Z = 8 - defined by sp3 hybrid orbitals (contrast: graphite sp2) Crystal shapes: - single-crystals mostly {111} - single-crystals often show spinel-law or other twining - common {110} shapes formed by resorption - also {100} or irregular shapes; often distorted, often rounded - often polycrystalline and fibrous diamond specimens - further: stacked growth, corrosion and abrasion features, deformations, etc.

Properties:

Gemology: Diamond

Physical properties: - perfect {111} cleavage - hardness 10 (Mohs) - density 3.52±0.02 g/cm3 - refractive index: 2.407 (red) − 2.451 (blue); dispersion 0.044 - ∆n = 0 (optically isotropic) - 5.5 eV band gap (note: insulator properties but wide bandgap semiconductor!) - colorless (theoretically); almost all possible colors observed

(four directions)

Composition:

Gemology: Diamond

→ Types of defects:

Chemical composition: β−C Trace elements: - up to ∼1000 ppm: , H, S - 0.1−100 ppm: , Mg, Al, Si, Ca, V, Cr, Mn, Fe, Co, Ni, Ag, Ba - <0.1 ppm: diverse metals Note: For several elemental species the nature as incorporated into the lattice or as sub-micrometer inclusions has not been clarified yet.

C-type defect: One nitrogen replacing carbon.

A-type defect: Two neighboring nitrogen.

B-type defect: Combination A + A + V

…and many more, including N3 (three nitrogen atoms surrounding a vacancy), GR1 (neutral vacancy), H3 (N + N + V = A + V); ca. 800 are known!

N B

Important for coloration. For instance “Cape yellow” (C) versus “Canary yellow” (N3).

→ Type classification based on N and B concentrations.

Composition-based classification: → Type classification based on N and B concentrations.

Gemology: Diamond

Formation:

Gemology: Diamond

Stability field: p-T ratio required for diamond formation only realized in craton areas Craton = old, stable parts of the continental lithosphere with deep (hundreds of km) roots Shield = basement rocks crop out at the surface Platform = basement rocks overlaid by sedimentary rocks / sediments Main diamond-bearing rocks: eclogite, peridotite

Peridotites: → Lherzolite: primarily olivine, with significant amounts of opx and cpx → Harzburgite: primarily of olivine and orthopyroxene, with small amounts of spl and gt → Dunite: mainly olivine; may contain significant amounts of chromite, pyroxene and spinel → Wehrlite: mainly orthopyroxene and clinopyroxene, with ol and hbl → Kimberlite: at least 35% olivine, with significant amounts of phlogopite, pyroxenes, carbonates, serpentine, diopside, monticellite, and garnet.

Diamond formation can be studied by analyzing diverse inclusions: Diamond crystals act as “containers” to transport mantle material to the Earth’s surface.

Gemology: Diamond

Use of diamond (including synthetic diamond and DLC): - Abrasives (drilling, cutting, grinding, polishing) - Optical devices - Semiconductor industry - Surface coatings - Earth sciences: DACs - Gems and jewelry; including currency

Production:

Gem diamond:

Gemology: Diamond

CIBJO: Confédération International de la Bijouterie, Joaillerie, Orfèvrerie des Diamantes, Perles et Pierres (International Confederation of Jewellery, Silverware, Diamonds and Stones) Diamond price determined mainly by the “four C”: Color Clarity Carat = weight (one carat = 0.2 grams) Cut

Note: Before GIA developed the D-Z Color Grading Scale, a variety of other systems were loosely applied. These included letters of the alphabet (A, B and C, with multiple A’s for the best stones), Arabic (0, 1, 2, 3) and Roman (I, II, III) numerals, and descriptions such as “gem blue” or “blue white.” The result of all these grading systems was inconsistency and inaccuracy. Because the creators of the GIA Color Scale wanted to start fresh, without any association with earlier systems, they chose to start with the letter D — a letter grade normally not associated with top quality.

Note: Many traditional but obsolete terms are also in use. As for instance “River” and “Top Wesselton” for the finest white colors of diamond, or “Frozen Horse Piss” for a rather ugly, pale yellow color. The use of terms such as “Cape yellow” and “Canary yellow” is generally accepted and appreciated , as these terms are related to certain types of defects.

Note: Low-quality stones quoted I1 − I3 are also often called “Pique” (P1 − P3).

Example: Improvement of the visual impression (“clarity”) by filling fractures with organic substances having high refractive index

Gem diamond:

Gemology: Diamond

Example: Improvement of the visual impression (“clarity”) by laser drilling of inclusions and filling boreholes with organic substances having high refractive index

Example: Decoloration (and increase of value) by HP-HT treatment (GE-POL technique)

To enhance the value of a stone, color, clarity, and cut-quality can be modified. - Filling of fractures - Removal of inclusions (for instance laser drilling) - Irradiation (change of color) - Coating with CVD diamond (color change / removal of fractures) - Implantation (color change) - Pressure-temperature-treatment (color change / decoloration)

Gemology: Diamond

Application of spectroscopy: Luminescence spectroscopy (CL = surface technique; PL may analyze inside): - unravelling growth zoning - Prime technique to get hints to enhancement procedures - Quality control (low-fluorescence criterion) for DAC Infrared absorption spectroscopy: - Quantification of nitrogen and other elemental defects (H, B, Ni) - Identification of, and hydrous species in, solid inclusions (genesis, provenance) - Type determination based on N and B defects Optical absorption spectroscopy: - Study of color causes by determination of elemental defects Example: Canary Yellow (C-center) vs. Cape Yellow (N3 center) - Possible hints to treatments (e.g. high concentration of H2 in green diamond) Raman spectroscopy: - Quick identification - Identification of organic fillers (drill-holes, fractures) - Identification of inclusions (genesis, provenance) - …more

Dresden Green (18th century, India) orig. ∼119,5 ct; green Wittelsbach-Graff (unknown, India) fac. 31.06 ct; deep blue

Hope (17th century, India) orig. ∼112 ct; dark grayish-blue

Orloff (unknown, India) 198,62 ct; bluish-greenish Golden Jubilee (1985, South Africa) orig. 755,5 ct; brown Cullinan (1905, Südafrika) 3106,75 ct; colorless

Gemology: Diamond

Exceptionally large and famous diamond stones:

Strawn-Wagner (Arkansas, 1990) orig. 3.09 ct; colorless

Lesedi La Rona – 1111/1109 ct (Botswana) so far unsold

Large uncut stone of Lucara Co. – 813 ct (Nov 2015, Botswana) sold at Sotheby’s for 63 Mill-USD

News in the web:

Name from the Tamil kuruvindam (dto. Sanskrit Kuruvinda) = Ruby (Woodward, 1714) - variety name Smirgel = microcrystalline corundum of metamorphic origin, often rich in iron oxide

Gemology: Corundum

Chemical composition: α-Al2O3, with traces of Cr, Fe, Ti, Mg, etc.

- Crystal structure: - trigonal, R3c - a0 = 4.75 Å; c0 = 12.98 Å; Z = 6 - AlO6 octahedrons, defined by oxygen planes perpendicular to the c-axis, small Al3+ ions in triple points above and below

Crystals mostly short prismatic (the often barrel-shaped) or (steep) bi-pyramidal, also thick tabular; crystal faces often “horizontally” striped (polysynthetic twinning perpendicular to the c-axis)

Generalities and properties:

Star corundum: Occurrence of numerous, needle-shaped inclusions (TiO2) causes the so- called asterism (six-rayed star; in the case of a twinned host two such stars)

Gemology: Corundum Physical properties: - virtually all colors: theoretically colorless (quite rare in nature), often gray, blue, red, green, yellow, black, or other; often patchy or other zoned coloration; no pleochroism; white streak - adamantine to vitreous luster - n = 1.76−1.77; ∆n = 0.008−0.010 (uniaxial negative) - no clear cleavage; conchoidal to uneven fracture; brittle - specific gravity 3.9−4.1 g/cm3

- Mohs hardness 9 Other properties: - chemically widely inert - thermally stable (melting point 2044 °C)

Ruby = red corundum colored by Cr3+ (Europe) / pink to blood-red colored corundum (US)

Gem corundum:

Sapphire = all other gem-quality corundum (e.g. pink sapphire, yellow sapphire, etc.) particularly important in the trade is BGY sapphire (blue-green-yellow) Padparadscha (also padparadscha sapphire) = orange corundum colored by beryllium name goes back to the Sanskrit padma ranga (lotus color), because the color is reminiscent to that of the lotus-flower blossom

Formation and production:

Gemology: Corundum

Occurrence: - primary magmatic: Al-rich, low-Si plutonites (syenites, nepheline syenites), pegmatites, ultramafic intrusives - primary metamorphic: schists, gneisses, marbles - secondary: eluvial detritus, and stream and beach gravels (main part of production)

Provenance analysis: - most difficult if just based on peculiarities of the chemical composition and isotopes, as formation milieus show overlapping conditions - should include inclusion studies and many more; nevertheless often not reliable

Exceptional stones: Liberty Bell ruby

Blue giant of the Orient

Neelanjali ruby Logan sapphire

Gemology: Beryl

Name from the Greek βήρυλλος (beryllos) = precious, sea-water-colored stone (Plinius, 77)

Generalities and properties:

Chemical composition: Al2Be3[Si6O18] - alkali elements (Rb, Cs) up to 7 wt% - water (up to 3 wt%) and carbon dioxide in channels , also noble gases (Ar, He) Crystal structure: - hexagonal, P6/mcc - a0 = 9.22 Å; c0 = 9.20 Å; Z = 2 - Si6O18 rings opening up channels perpendicular to the c-axis Crystals mostly prismatic, with pronounced striations parallel to the c-axis - often corroded surface shapes

Trapiche: - Spanish word for the grinding wheel used to process sugarcane in the region of Colombia, South America - spoked texture may form due to oriented intergrowth of emerald and albite - spoked texture may form due to oriented incorporation of dark (graphitic) pigments

Generalities and properties:

Gemology: Beryl

Color varieties: - Emerald (deep green; colored by Cr and sometimes also V) - name from emeraude (Middle English), from ésmeraude (Old French), from esmeraldus (Medieval Latin), from smaragdus (Latin) from σμάραγδος (smaragdos, Greek) ="green gem“ - very often rich in inclusions and fractured; hence often poor transparency - Aquamarine (blue, cyan, ocean greenish-blue) - Heliodor (also “golden beryl”; between pale yellow to intense golden) - Goshenite (colorless) - Morganite (also “pink emerald” or “pink beryl”; light pinkish to rose-colored) - Red beryl (former name “bixbite” was deprecated by CIBJO because of the similarity to the mineral bixbyite; rich red color) Note: “Raspberry beryl” is not a beryl but the newly identified, beryl-group mineral pezzottaite Cs(Be2Li)Al2Si6O18

Generalities and properties:

Gemology: Beryl

Physical properties: - many colors (as seen) except brown-black; weak to distinct pleochroism - vitreous luster - n = 1.56−1.60; ∆n = 0.004 (uniaxial negative) - imperfect cleavage (0001); conchoidal to irregular fracture; brittle - specific gravity 2.6−2.9 g/cm3

- Mohs hardness 7.5−8

Formation and production: Occurrence: - primary magmatic: (granitic) pegmatites - primary post-magmatic: formed by fluid activity in limestones (Colombia) - primary metamorphic: schists (Ural mountains, Habach valley) - secondary: gravels (then rounded) Main production (emerald): - Colombia - Brazil - Zambia - Russia - Zimbabwe - Madagascar, Pakistan, Canada, Afghanistan

Exceptional stones: Chalk emerald - originating from Colombia - faceted 37.82 ct; set in ring with 60 pear-shaped diamonds - 1972 donated by O. Roy Chalk to the Smithsonian Natural History Museum

Gemology: Beryl

Moor with emerald level - emerald specimen (64×29×31 cm); gift of 1581 (uncertain origin) - sculptural work 1724 - on display in the green Vault, Dresden

Name after chrysós = gold and beryllos = clear crystal (Werner, 1790)

Crystal structure: - orthorhombic with a0 = 5,48 Å, b0 = 4,43 Å, c0 = 9,41 Å (Z = 4) - isotyp with fayalite-forsterite

Gemology: Chrysoberyl

Chemical composition: Al2BeO4, always with Fe3+ content (<6 wt%), often Ti (<3 wt%) - Alexandrite = Cr3+-bearing (<0.4 wt%), with color change: green coloration in daylight, purplish-reddish in artificial (incandescent) light

Crystals short prismatic to tabular; typically with striations; very often twinned (120°)

Physical properties: - predominantly greenish (all shades); also yellowish, brownish; clear pleochroism - color-change to red in artificial light if Cr-bearing - vitreous luster - n = 1.745−1.754; ∆n = 0.009 (biaxial positive) - cleavage parallel to {010}; conchoidal to uneven fracture; brittle - specific gravity around 3.8 g/cm3

- Mohs hardness 8.5 Chrysobely cat’s eye = Cymophane (chatoyancy due to included parallel mineral fibers) Formation: in pegmatites (late stage: pegmatitic-pneumatolytic), granites, contact- metamorphic slances; often in gravels Main production: Russia (Urals), India, Madagascar, Tanzania, Sri Lanka

Exceptional stones: Smithsonian alexandrite - originating from Sri Lanka - faceted 65.08 ct (cuhion mixed cut)

Gemology: Chrysoberyl

Naleem alexandrite - faceted 112 ct, unconfirmed! - allegedly in the collection of Marhoom al-Hajj Naleem, Sri Lankan businessman Unnamed alexandrite - collection of private owner in Japan, certified by State Gem Cooperation; listed as the world’s largest faceted alexandrite in the Guinness Book of Records - faceted 142 ct, unconfirmed! - if confirmed, estimated value would exceed US$ 100,000,000

Gemology: Garnet-group minerals

General formula A2+3 B3+

2 [SiO4]3

„Pyralspite“: Pyrope Mg3Al2[SiO4]3 Almandine Fe3Al2[SiO4]3 Spessartine Mn3Al2[SiO4]3

„Ugrandite“: Uvarovite Ca3Cr2[SiO4]3 Grossular Ca3Al2[SiO4]3 Andradite Ca3Fe2[SiO4]3

further: Schorlomite = Ca-Ti garnet Melanite = Ti-bearing andradite, dark Demantoite = greenish andradite Topazolite = greenish yellow andradite Hessonite = red (Fe3+) grossular Tsavorite = green (Cr and/or V) grossular

A = Mg, Fe2+, Mn, Ca B = Al, Fe3+, Cr (and many minor and trace elements)

Synthetic garnet = no natural analog; important industrial uses YAG = Yttrium aluminium garnet GGG = Gadolinium gallium garnet TGG = Terbium gallium garnet GSGG = Gadolinium scandium gallium garnet

- Crystal structure: cubic, Ia3d a0 = 11,45 Å (pyrope); a0 = 11,53 Å (almandine); a0 = 11,62 Å (spessartine); a0 = 11,85 Å (grossular); a0 = 11,97 Å (uvarovite); a0 = 12,04 Å (andradite) - A ions are eight-coordinated; along with BO6 octahedrons and SiO4 tetrahedrons

Gemology: Garnet-group minerals

General formula A2+3 B3+

2 [SiO4]3

Crystals mostly rhombic dodecahedron {110} or icositetrahedron {211}, less commonly combined with {111} or {100} Properties: - wide range of colors; vitreous to resinous luster - refraction 1.71−1.88; no birefringence; no pleochroism - indistinct cleavage, uneven fracture; Mohs hardness 6.5−7.5 - specific gravity 3.6−4.2 g/cm3

Formation: - magmatic: in kimberlites and peridotites (esp. pyrope, majorite) in granites and granitic pegmatites (esp. almandine, spessartine) - metamorphic: regional metamorphism (schists, eclogite: esp. almandine, grossular contact metamorphism silicatic (almandine, spessartine) contact metamorphism carbonatic (grossular, andradite) - sedimentary: occurring in gravels - may decompose to form chlorite/biotite

Gemology: Topaz

General formula Al2[(F,OH)2/SiO4] - Name from the Greek Τοπάζιος (Τοpáziοs), the ancient name St. John’s Island (Red Sea), where in ancient times a yellow stone was mined (as we know now, probably yellowish peridot) - Often rich in fluid inclusions - Imperial topaz: Yellow, orange, or rarely pink topaz of gem quality - Gold topaz: Rather yellowish “imperial” topaz - Mystic topaz: Colorless topaz, coated artificially to create a rainbow effect

Crystal structure: - orthorhombic, Pbnm - a0 = 4,65 Å; b0 = 8,80 Å; c0 = 8,40 Å - Al(F,OH)6 octahedrons and isolated SiO4 tetrahedrons

Crystals often with {001}, {010}, {110}, {120}, {021}; predominantly short prismatic habit; prisms often with striations parallel to the c-axis; occasionally “corroded” surface features

Gemology: Topaz Properties: - wide range of colors: Colorless (if no impurities), blue (often irradiated!), brown, orange, gray, yellow, green, pink and reddish pink - vitreous luster - refraction 1.60−1.64; ∆n = 0.010; weak pleochroism - perfect (001) cleavage; uneven to subconchoidal fracture - Mohs hardness 8 - specific gravity 3.49−3.57 g/cm3

Formation: - late magmatic in (granitic) pegmatites (Ural Mountains, Brazil, etc.) - late magmatic in vapor cavities in rhyolite lava flows (Topaz Mountain, Utah) - early post-magmatic in pneumatolytic nests and veins (topaz breccia, Schneckenstein)

- Name from the Persian zargun (زرگون), meaning gold-colored stone. According to other sources from the old name of Sri Lanka (Ceylon) - Minor element: Hf (0.5−2.0 wt%); trace elements: U, REE, Nb, Ta - Hyacinth = red colored gem zircon (after the hyacinthus flower) - Jargoon = light colored zircon of gem quality (ambiguously used) - Self-irradiation damage due to U- and Th-decay: traditional classification into high-zircon and low-zircon

Gemology: Zircon

General formula ZrSiO4

Crystal structure: - tetragonal, I41/amd - a0 = 6.59 Å; c0 = 5,94 Å - Zr eight-coordinated; isolated SiO4 tetrahedrons

Crystals mostly short prismatic

Gemology: Zircon Properties: - wide range of colors: Reddish brown, yellow, blue (treated!), gray, colorless (if no impurities; rare), green (“low-zircon”, typically 0.3−0.9 wt% U, radioactive gem!), dark brown to black (typically when metamict: “malacon”, “cyrtholithe”) - vitreous to adamantine luster (metamict: greasy) - refraction 1.92−2.01 (1.75 when metamict); ∆n = 0.055 (zero when metamict); weak pleochroism - perfect (001) cleavage; uneven to subconchoidal fracture - Mohs hardness 7.5 (lower when metamict) - specific gravity 4.7 g/cm3 (down to ca. 3.8 g/cm3 when metamict) Formation: - early formation as accessory component in intermediate to acidic igneous rocks - large crystals in pegmatites - often found in gravels

Use of gem-quality zircon in research: - natural reference material in U−Pb geochronology (no reliable possibility to synthesize synthetic ZrSiO4 doped with Pb) - natural reference material in (U+Th)/He thermochronology (dto) - reference material in petrogenesis: Oxygen, hafnium, lithium stable isotopes

- Origin of name unclear; presumably from Latin pæderot, describing a greenish opal - Isomorphism Fe2+−Mg2+; minor elements: Ni (<0.6 wt%), Co, Cr, Mn, Al, Ti - Orthorhombic nesosilicate; crystals rarely idiomorphic; - Formation in basaltic rocks, occasionally contact-metasomatic (forsteritic olivine) - Occasional replacement of emerald and other green gems; however Mohs hardness 6.5−7 Formation: basic igneous rocks; also contact-metasomatic

Gemology: “Peridot”

General formula (Mg,Fe)2SiO4

Gem-quality variety of olivine-group minerals (fayalite−forsterite); also chrysolite

Gemology: Tourmaline-group minerals

General formula X Y3 Z6 [(BO3)3 T6 O18 (OH,O)3 (OH,F,O)] X=(Ca, Na, K, ) Y=(Mg, Li, Al, Mn, Fe2+, Fe3+, V, Cr, Ti, Cu, ), Z=(Al, Mg, Cr, V, Fe3+, Ti), T=(Si, Al, B, Be) Name from the Sinhalese word “turmali” (තුරම), applied to various gems from Sri Lanka

Schorl Na Fe32+ (Al,Fe3+)6 [(OH)4 / (BO3)3 / Si6O18]

Dravite Na Mg3 Al6 [(OH)4 / (BO3)3 / Si6O18] Elbaite Na (Li,Al)3 Al6 [(OH,F)4 / (BO3)3 / Si6O18] …etc. (ca. 30 tourmaline-group minerals are recognized by the IMA) Achroite = colorless elbaite; Indigolite = light blue to bluish green elbaite; Rubellite = pinkish to red elbaite; Verdelite (or “Brazilian emerald”) = green elbaite Often multi-colored crystals! Watermelon tourmaline = tri-colored crystals; center pink − pale − rim green Paraiba tourmaline = any diamagnetic, blue tourmaline (colored by copper; in contrast to magnetic, blue tourmaline whose color is due to iron); named after Mina da Batalha, Paraiba, Brazil

Properties: - wide range of colors: Most commonly black, but can range from colorless, brown, violet, yellow, orange, blue, red, green, pink, or bi-colored, or even tri-colored. - vitreous, sometimes resinous luster - refraction 1.61−1.67; ∆n up to -0.040; often strong pleochroism - indistinct cleavage; uneven fracture; brittle - Mohs hardness 7−7.5 - specific gravity 2.8−3.3 g/cm3

Formation: - late magmatic in (granitic) pegmatites - early post-magmatic in supercritical (i.e. pneumatolytic) parageneses - occurs in gravels, because resistant to weathering

Gemology: Tourmaline-group minerals Crystal structure: - trigonal, R3m - a0 = 15,84 Å (elbaite) − 16,07 Å (schorl) c0 = 7.10 Å (elbaite) − 7.30 Å (schorl) - six-membered rings of SiO4 tetrahedrons, forming polar Si6O18 rings

Crystals mostly elongated prisms, nearly always with striations parallel to the c-axis; aggregates often radiating

Gemology: “Jade”

Ornamental material (rock), applied to two main types of metamorphic rocks / minerals: (1) Jadeitic jade = microcrystalline interlocking matrix of Na-Al pyroxene. Named after the Spanish term piedra de ijada or “loin stone”, from its reputed efficacy in curing ailments of the loins and kidneys. (2) Nephritic jade (nephrite) = fibrous matrix of actinolithic clinoamphibole [tremolite-type (Ca-Mg) and/or ferroactinolite-type (Ca-Mg-Fe)]. Derived from lapis nephriticus, the Latin version of the Spanish piedra de ijada.

Mostly green colors, commonly ranging from pale grayish to deep green Further: Mutton fat jade = creamy white form of nephritic jade, known from China Colorations of jadeitic jade include blue, lavender-mauve, pink, and emerald- green (the latter being most highly priced)

Used from prehistoric periods for hardstone carving, especially famous in eastern Asia

Gemology: Other silicates

Phenakite, Be2SiO4, trigonal Euclase, BeAlSiO4(OH), monoclinic Sillimanite, Al2OSiO4, orthorhombic (“fibrolite”) Andalusite, Al2OSiO4, orthorhombic (note: var. chiastolite, pigmented) Kyanite, Al2OSiO4, triclinic Cordierite, (Mg,Fe)2 Al3 [AlSi5O18], orthorhombic Clinohumite, (Mg,Fe2+)9(SiO4)4(F,OH)2, monoclinic Titanite, CaTiSiO5, monoclinic Zoisite, Ca2 Al Al2 [O/OH/SiO4/Si2O7], monoclinic (var. tanzanite) Benitoite, BaTiSi3O9, hexagonal

Non-silicate gem materials: Opal Opal = hydrated amorphous silica: SiO2 · n H2O (water content may vary between 1 wt% and up to 20-30 wt%) Name adapted from the Sanskrit word úpala (= precious gem). Amorphous; consists of a multitude of silica spheres (diameters on the order of the wavelength of light). Common opal = some degree of disorder and size distribution Gem opal = ordered, close-packed spheres (of uniform or at least similar diameter) Precious opal = variable interplay of internal colors Fire opal = reddish-orangy Harlequin opal = intense play of colors with black background Hyalite opal = rather colorless, transparent Non-crystalline opal = contrast to truly amorphous opal (“opal-A”): formation of small domains of silica polymorphs including cristobalite (“opal-C”) and tridymite (“opal-CT”, “opal-T”) Syntheses produced by Kyocera and Inamori (Japan; technique goes back to Gilson, 1974) Note: in the sale often composite opals (doublets, triplets)

5 μm

Non-silicate gem materials: Quartz Chemical formula: α−SiO2 (trigonal-trapezohedral framework of tetrahedrons) Varieties of macro-crystalline quartz: a) colored due to defects (trace elements or irradiation) - Rock crystal - Amethyst - Citrine - Smoky quartz - Morion - Prasiolite (= green quartz) b) colored because of finely dispersen inclusions - Prasem (by actinolite) - Rose quartz (by dumortierite) - Blue quartz (= Saphirquarz; by krokydolite) - Milky quartz (by fluid inclusions) Varieties of micro-crystalline quartz: - Chalcedony (fibers along [1120]) - Quartzine (fibers along [0001]) - Agate (incl. onyx, moss agate, snow agate, and many more; often coloured!) - Jasper, carnelian - Tiger‘s eye, Aventurine, Rutilated quartz Not genuine varieties: Aqua aura, mystic quartz (Ti coated)

-

Non-silicate gem materials: Spinel Chemical formula: MgAl2O4 with some Fe (<3.5 wt%); also Cu, Cr Sapphire spinel = blue due to Fe2+

Origin of name: spinnós (= to sparkle) Belongs to cubic oxides with the general formula A2BO4

Aluminum spinels Spinel Al2MgO4 (more exactly Magnesian spinel) Hercynite Al2FeO4 Gahnite Al2ZnO4 Iron(III) spinels Magnetite Fe3+(Fe3+Fe2+)O4 Franklinite Fe2ZnO4 Chromium spinels Magnesian chr. Cr2MgO4 Chromite Cr2FeO4 Ti and V spinels Ulvöspinel (Fe2+Ti4+)Fe2+O4 (syn. Ulvite) Regular spinels: A2

[6] B[4] O4

Inverse spinels: (A[6] B[6]) A[4] O4 5 μm

Non-silicate gem materials: Spinel Chemical formula: MgAl2O4 Formation: - magmatic in deep-cryst or upper-mantle rocks (peridotites) - gem spinel contact metamorphic - often in gravels - note: many syntheses (with or wirhout natural analogs) Main production: Cambodia, Myanmar, Sri Lanka and Thailand (also Afghanistan, Australia, Brazil, Madagascar, and USA)

cubic, a0 = 8,09 Å (Z = 8) Crystals mostly {111} , often twinned

Example: - 170 ct “Black Prince’s Ruby”, set into the British Imperial State Crown, above the 317 ct Cullinan II diamond - 361 ct “Timur ruby” (also Khiraj-i-alam, "Tribute to the World") is an unfaceted, polished red spinel gemstone set in a necklace in 1853, part of the British Crown Jewels. It is named after the ruler Timur. It was believed to be a ruby until 1851.

Non-silicate gem materials: Spinel Example: - 170 ct “Black Prince’s Ruby”, set into the British Imperial State Crown, above the 317 ct Cullinan II diamond - 361 ct “Timur ruby” (also Khiraj-i-alam, "Tribute to the World") is an unfaceted, polished red spinel gemstone set in a necklace in 1853, part of the British Crown Jewels. It is named after the ruler Timur. It was believed to be a ruby until 1851.

Non-silicate gem materials: Others Taaffeite, Mg3Al8BeO16

Sinhalite Mg(Al,Fe)BO4

Brasilianite, NaAl3[(OH)2|PO4]2

Malachite Cu2[(OH)2|CO3] Jeremejevite, Al6B5O15(F,OH)3

Apatite, Ca5[(F,Cl,OH)/(PO4)3] Turquoise, CuAl6(PO4)4(OH)8(H2O)4

…and many more

Biogenic gem materials: Pearls Amber (fosslized tree resin) Corals Ivory (elephant may be critical; mammouth OK) Mother of pearl Pearls

Pearls = gems formed directly through biological processes, i.e. biomineralization Note: Virtually all mollusks with a calcium-carbonate shell are able to produce pearls (bivalves, gastropods and cephalopods). However, not all pearls are attractive enough to be used in jewelry Classification of pearls according to the formation (A): natural versus cultured Type I: Natural pearls (NPs) are secreted accidentally by mollusks without any human intervention Type II: Cultured pearls (CPs) are produced after transplantation by man of a tissue (with or without the implantation of a bead) Note: In the today’s market there are 99.8% cultured pearls and 0.2% natural pearls. The vast majority is derived from bivalve mollusks Classification of pearls according to the formation (B): salt (SW) versus fresh water (FW)

Biogenic gem materials: Pearls

Production of cultured pearls Step 1: Growth of mollusks (12−24 months), either in hatcheries (more homogeneous samples) or wild; then collection for step 2… Step 2: Grafting (with bead or without bead) Step 3: Growth of pearls (18 months to several years) Step 4: Collection of pearls

Biogenic gem materials: Pearls

Generalities / History - 14th century, China: first successful cultivation ("blister" pearls) - 19th century, Mexico: first successful experiments to cultivate round pearls - ca. 1920: Patent for big-scale cultivation of round pearls (Mikimoto) Akoya pearls: - SWCP, cultivated in Pinctada fucata, mainly in Japan and China, also Vietnam and Korea - predominantly beaded, mainly from 7 to 9 mm (rarely up to 12 mm) - mainly white to cream, rarely yellowish and bluish Tahitian pearls: - SWCP, cultivated in Pinctada margaritifera, mainly in French Polynesia, Cook Islands, Fiji - beaded (non-beaded by accident), from 7 to 12 mm (rarely up to 16 mm) - most commonly black to light grey, also other tones (and secondary overtones)

FWCPs: - 96% of pearls in the market are freshwater cultured pearls - in 2006, the world’s main producer China has grown 1500 t of FWCP (800 t were suitable for jewelry; about 75 t were of superior quality). for comparison: Akoya ca. 20 t, Tahiti ca. 15 t top quality - the vast majority of FWCPs are cultivated by tissue graft transplantation (non-beaded) - mollusks used nowadays include: Hyriopsis cumingi, H. schlegeli (Biwa cultured pearls), H. schlegeli x cumingi (Kasumiga cultured pearls) - from 6 to 8 mm (rarely up to 12 mm)

Biogenic gem materials: Pearls

NPs (examples): - Melo pearls (non-nacreous; from Melo ssp.) - Abalone pearls (non-nacreous; from Haliotis sp.) - Queen conch pearls (non-nacreous; from Strombus gigas)

Assignment and valuation of pearls - Type of formation (NP versus CP); if cultured, bead or non-bead; if bead, which type and which thickness of nacre? - Formation milieu (SW versus FW) - Host animal (mollusk or other; CITES protected?) Note: more than 99.9% of all pearls are derived from bivalve mollusks - Origin (e.g. Japanese pearls versus Chinese pearls versus …) - Color treated?

Biogenic gem materials: Pearls

Growth milieu: SW versus FW - Note again: 96% of all pearls in the market are FWCP - main producer: China

Beaded vs. non-beaded: - SWCP almost exclusively with bead; most FWCPs are without bead - Methods: Observation into drillhole; Radiography (X-raying); X-ray CT

Naturally colored vs. color-treated: - Methods: Microscopic observation; analysis of pigment material

Provenance analysis: rarely possible