Precious & Semi-Precious Gemstone Compendium

Notes on Classification

  • Precious (historical/market): diamond, ruby, sapphire, emerald. Many so-called semi-precious gems can be rarer or more valuable in fine quality. Use the labels for navigation, not hierarchy.
  • Organics/Mineraloids included: pearl, coral, amber, jet, ammolite, opal (mineraloid).

Every entry includes:

  • Category (precious/semi-precious)
  • Group/Variety
  • Chemical Formula
  • Mohs Hardness
  • Crystal System
  • Colors
  • Refractive Index (RI)
  • Specific Gravity (SG)
  • Common Treatments
  • Notable Localities
  • Etymology
  • Definition

Precious Stones

Diamond

  • Category: Precious
  • Group: Native element
  • Formula: C (pure carbon)
  • Mohs: 10
  • Crystal System: Isometric (cubic)
  • Colors: Colorless to fancy colors (yellow, pink, blue, green, red, black)
  • RI: 2.42
  • SG: ~3.52
  • Treatments: HPHT (high pressure, high temperature), irradiation, laser drilling, fracture filling
  • Localities: Botswana, Russia, Canada, South Africa, Australia
  • Etymology: From Ancient Greek adámas (ἀδάμας), “unconquerable” or “invincible,” referring to its hardness.
  • Definition: The hardest known natural mineral, composed entirely of carbon atoms in a cubic crystal lattice, valued for its brilliance, durability, and rarity; used in jewelry and for industrial cutting and drilling.

Emerald (Beryl variety)

  • Category: Precious
  • Group: Beryl
  • Formula: Be₃Al₂(SiO₃)₆ with Cr and/or V for green color
  • Mohs: 7.5–8
  • Crystal System: Hexagonal
  • Colors: Green to bluish-green
  • RI: 1.57–1.59
  • SG: 2.67–2.78
  • Treatments: Oil or resin filling to reduce appearance of fractures (common)
  • Localities: Colombia, Zambia, Brazil, Ethiopia, Afghanistan
  • Etymology: From Old French esmeraude, from Latin smaragdus, from Ancient Greek smáragdos, “green gem.”
  • Definition: Green gemstone variety of beryl prized for vivid color and rarity; inclusions, often called jardin, are common and accepted in the trade.

Ruby (Corundum variety)

  • Category: Precious
  • Group: Corundum
  • Formula: Al₂O₃ with Cr (chromium) giving red color
  • Mohs: 9
  • Crystal System: Trigonal
  • Colors: Red, from pinkish-red to deep crimson (“pigeon’s blood”)
  • RI: 1.76–1.77
  • SG: ~4.0
  • Treatments: Heat treatment to improve color and clarity; flux healing; lead-glass filling (must be disclosed)
  • Localities: Myanmar, Mozambique, Thailand, Sri Lanka, Madagascar
  • Etymology: Latin rubeus/ruber, “red.”
  • Definition: Red variety of corundum, historically one of the most valuable gemstones, symbolizing passion, vitality, and wealth.

Sapphire (Corundum variety)

  • Category: Precious
  • Group: Corundum
  • Formula: Al₂O₃ with trace elements (Fe, Ti, etc.) for color
  • Mohs: 9
  • Crystal System: Trigonal
  • Colors: All colors except red; blue most prized; padparadscha is pink-orange
  • RI: 1.76–1.77
  • SG: ~4.0
  • Treatments: Heat to improve color; beryllium diffusion for exotic colors (must be disclosed)
  • Localities: Sri Lanka, Kashmir, Myanmar, Madagascar, Montana (USA)
  • Etymology: Old French saphir, from Latin sapphirus, from Greek sappheiros, possibly originally referring to lapis lazuli.
  • Definition: Durable gemstone variety of corundum, valued for its hardness, brilliance, and wide color range, traditionally symbolizing wisdom and royalty.

Semi-Precious

Alexandrite (Chrysoberyl)

  • Category: Semi-Precious
  • Group: Chrysoberyl
  • Formula: BeAl₂O₄ + Cr
  • Mohs: 8.5
  • Crystal System: Orthorhombic
  • Colors: Green (daylight) to red (incandescent light)
  • RI: 1.745–1.755
  • SG: 3.73
  • Treatments: Rarely treated; synthetics exist
  • Localities: Russia, Sri Lanka, Brazil, East Africa
  • Etymology: Named for Tsar Alexander II of Russia.
  • Definition: Rare color-change gemstone valued for its dramatic shift from green to red under different lighting.

Amazonite (Microcline Feldspar)

  • Category: Semi-Precious
  • Group: Feldspar
  • Formula: KAlSi₃O₈
  • Mohs: 6–6.5
  • Crystal System: Triclinic
  • Colors: Green to bluish-green
  • RI: 1.522–1.530
  • SG: 2.56–2.58
  • Treatments: Usually untreated
  • Localities: Russia, USA, Madagascar, Brazil
  • Etymology: Named after the Amazon River (no deposits there).
  • Definition: An opaque to translucent green feldspar used in jewelry and decorative objects.

Amber

  • Category: Semi-Precious (Organic)
  • Group: Fossilized plant resin
  • Formula: Approx. C₁₀H₁₆O
  • Mohs: 2–2.5
  • Crystal System: Amorphous
  • Colors: Yellow to orange-brown, sometimes green, blue, red
  • RI: 1.54
  • SG: 1.05–1.10
  • Treatments: Heat, oil, reconstitution from fragments
  • Localities: Baltic region, Dominican Republic, Myanmar
  • Etymology: From Arabic ʿanbar.
  • Definition: Fossilized tree resin often containing inclusions; valued for jewelry and ornament.

Amethyst (Quartz)

  • Category: Semi-Precious
  • Group: Quartz
  • Formula: SiO₂
  • Mohs: 7
  • Crystal System: Trigonal
  • Colors: Violet to purple
  • RI: 1.544–1.553
  • SG: 2.65
  • Treatments: Heat, irradiation
  • Localities: Brazil, Uruguay, Zambia, Russia, USA
  • Etymology: Greek améthystos (“not intoxicated”).
  • Definition: A violet variety of quartz historically believed to prevent drunkenness.

Ammolite

  • Category: Semi-Precious
  • Group: Aragonite (fossilized ammonite shell)
  • Formula: CaCO₃
  • Mohs: 3.5–4.5
  • Crystal System: Orthorhombic
  • Colors: Iridescent red, green, blue, violet
  • RI: 1.52–1.68
  • SG: 2.60–2.85
  • Treatments: Stabilization with resin
  • Localities: Canada, USA
  • Etymology: From “Ammon” (horned deity) + “-lite” (stone).
  • Definition: Fossilized shell gemstone with opal-like color play.

Andalusite

  • Category: Semi-Precious
  • Group: Nesosilicate
  • Formula: Al₂SiO₅
  • Mohs: 6.5–7.5
  • Crystal System: Orthorhombic
  • Colors: Brown, green, yellow; pleochroic
  • RI: 1.629–1.650
  • SG: 3.13–3.16
  • Treatments: Usually untreated
  • Localities: Spain, Brazil, Sri Lanka, USA
  • Etymology: Named after Andalusia, Spain.
  • Definition: A pleochroic gemstone showing different colors from different angles, valued for its optical effects.

Apatite

  • Category: Semi-Precious
  • Group: Phosphate mineral
  • Formula: Ca₅(PO₄)₃(F,Cl,OH)
  • Mohs: 5
  • Crystal System: Hexagonal
  • Colors: Green, blue, yellow, violet, colorless
  • RI: 1.628–1.649
  • SG: 3.16–3.22
  • Treatments: Heat to enhance color
  • Localities: Brazil, Mexico, Madagascar, Myanmar
  • Etymology: Greek apátē (“deceit”), for its frequent misidentification.
  • Definition: A brittle gemstone in vivid colors, used mainly in collectors’ jewelry.

Aquamarine (Beryl)

  • Category: Semi-Precious
  • Group: Beryl
  • Formula: Be₃Al₂(SiO₃)₆ + Fe
  • Mohs: 7.5–8
  • Crystal System: Hexagonal
  • Colors: Light blue to bluish-green
  • RI: 1.577–1.583
  • SG: 2.68–2.74
  • Treatments: Heat to enhance blue hue
  • Localities: Brazil, Nigeria, Madagascar, Pakistan
  • Etymology: Latin aqua marina, “sea water.”
  • Definition: A transparent blue beryl prized for its clarity and sea-like colors.

Aventurine

  • Category: Semi-Precious
  • Group: Quartz or feldspar with inclusions
  • Formula: SiO₂ (with mica, hematite, or goethite)
  • Mohs: ~7
  • Crystal System: Trigonal (quartz) or monoclinic (feldspar)
  • Colors: Green, orange, brown, yellow, blue
  • RI: ~1.544–1.553
  • SG: ~2.65
  • Treatments: Dyeing to enhance color
  • Localities: India, Brazil, Russia
  • Etymology: Italian a ventura, “by chance,” after goldstone glass discovery.
  • Definition: Sparkling quartz or feldspar variety with aventurescence from reflective inclusions.

Azurite

  • Category: Semi-Precious
  • Group: Carbonate mineral
  • Formula: Cu₃(CO₃)₂(OH)₂
  • Mohs: 3.5–4
  • Crystal System: Monoclinic
  • Colors: Deep blue
  • RI: 1.720–1.850
  • SG: 3.77–3.89
  • Treatments: Stabilization for softness
  • Localities: Morocco, USA, Australia, Namibia
  • Etymology: From azure, Persian lazhward, “blue.”
  • Definition: A soft, vivid blue copper carbonate mineral, often found with malachite.

Benitoite

  • Category: Semi-Precious
  • Group: Cyclosilicate
  • Formula: BaTiSi₃O₉
  • Mohs: 6–6.5
  • Crystal System: Hexagonal
  • Colors: Sapphire-blue, colorless, pink
  • RI: 1.757–1.804
  • SG: 3.65
  • Treatments: None
  • Localities: San Benito County, California
  • Etymology: Named for its discovery locality in California.
  • Definition: Rare blue barium titanium silicate, California’s state gem.

Bloodstone (Heliotrope)

  • Category: Semi-Precious
  • Group: Chalcedony
  • Formula: SiO₂ with iron oxide inclusions
  • Mohs: 6.5–7
  • Crystal System: Cryptocrystalline
  • Colors: Green with red spots
  • RI: ~1.530
  • SG: ~2.61
  • Treatments: Usually untreated
  • Localities: India, Brazil, Australia
  • Etymology: Greek hēliotropion, “sun turner,” from ancient lore.
  • Definition: Green chalcedony with red jasper spots, traditionally used as a healing stone.

Carnelian (Chalcedony)

  • Category: Semi-Precious
  • Group: Quartz (cryptocrystalline)
  • Formula: SiO₂
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Orange to reddish-brown
  • RI: 1.535–1.539
  • SG: 2.58–2.64
  • Treatments: Heat to deepen color; dyeing
  • Localities: India, Brazil, Uruguay, USA
  • Etymology: Latin carneus, “flesh-colored.”
  • Definition: Translucent orange-red chalcedony used in cameos, seals, and jewelry since antiquity.

Chalcedony

  • Category: Semi-Precious
  • Group: Quartz (cryptocrystalline)
  • Formula: SiO₂
  • Mohs: ~7
  • Crystal System: Cryptocrystalline (trigonal microstructure)
  • Colors: White, blue, gray, pink, and more
  • RI: 1.530–1.543
  • SG: 2.58–2.64
  • Treatments: Dyeing, heat
  • Localities: Brazil, USA, India, Madagascar
  • Etymology: From Chalcedon, ancient port in Asia Minor.
  • Definition: A microcrystalline variety of quartz in many colors, used in beads, carvings, and decorative objects.

Chrysoberyl

  • Category: Semi-Precious
  • Group: Oxide mineral
  • Formula: BeAl₂O₄
  • Mohs: 8.5
  • Crystal System: Orthorhombic
  • Colors: Yellow, green, brown; chatoyant in cat’s-eye form
  • RI: 1.746–1.755
  • SG: 3.73
  • Treatments: Rarely treated
  • Localities: Brazil, Sri Lanka, East Africa
  • Etymology: Greek chrysos “gold” + beryllos “beryl.”
  • Definition: Durable gemstone with strong brilliance, includes rare alexandrite and cat’s-eye varieties.

Chrysocolla

  • Category: Semi-Precious
  • Group: Hydrated copper silicate
  • Formula: (Cu,Al)₂H₂Si₂O₅(OH)₄·nH₂O
  • Mohs: 2–4
  • Crystal System: Amorphous to monoclinic
  • Colors: Blue-green
  • RI: ~1.46–1.57
  • SG: 2.0–2.4
  • Treatments: Stabilization for jewelry use
  • Localities: USA (Arizona, New Mexico), Chile, Peru
  • Etymology: Greek chrysos “gold” + kolla “glue.”
  • Definition: Soft blue-green copper mineral used in ornamental stones, often mixed with quartz for durability.

Chrysoprase

  • Category: Semi-Precious
  • Group: Quartz (chalcedony)
  • Formula: SiO₂ + Ni (nickel)
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Apple-green to deep green
  • RI: 1.534–1.540
  • SG: 2.58–2.64
  • Treatments: Dyeing
  • Localities: Australia, Brazil, USA, Russia
  • Etymology: Greek chrysos “gold” + prason “leek.”
  • Definition: Bright green chalcedony valued for its translucency and even color.

Citrine

  • Category: Semi-Precious
  • Group: Quartz
  • Formula: SiO₂
  • Mohs: 7
  • Crystal System: Trigonal
  • Colors: Yellow to golden brown
  • RI: 1.544–1.553
  • SG: 2.65
  • Treatments: Heat-treated amethyst or smoky quartz
  • Localities: Brazil, Spain, Madagascar, USA
  • Etymology: French citron, “lemon.”
  • Definition: Transparent yellow quartz used as a more affordable alternative to yellow sapphire or topaz.

Coral

  • Category: Semi-Precious (Organic)
  • Group: Calcium carbonate (aragonite) skeletons
  • Formula: CaCO₃
  • Mohs: 3–4
  • Crystal System: Orthorhombic microstructure
  • Colors: Red, pink, white, orange
  • RI: ~1.486–1.658
  • SG: ~2.60–2.70
  • Treatments: Dyeing, waxing, resin impregnation
  • Localities: Mediterranean Sea, Japan, Taiwan
  • Etymology: Latin corallium.
  • Definition: Skeleton of marine organisms, polished for jewelry and decorative items.

Diamond

  • Category: Precious
  • Group: Native element
  • Formula: C
  • Mohs: 10
  • Crystal System: Isometric
  • Colors: Colorless to fancy colors
  • RI: 2.42
  • SG: ~3.52
  • Treatments: HPHT, irradiation, laser drilling, fracture filling
  • Localities: Botswana, Russia, Canada, South Africa
  • Etymology: Greek adámas, “unconquerable.”
  • Definition: The hardest known natural mineral, composed entirely of carbon, prized as a gemstone and industrial material.

Diaspore

  • Category: Semi-Precious
  • Group: Oxide hydroxide mineral
  • Formula: AlO(OH)
  • Mohs: 6.5–7
  • Crystal System: Orthorhombic
  • Colors: Color-change from green to pink-red
  • RI: 1.702–1.750
  • SG: 3.39–3.45
  • Treatments: Usually untreated
  • Localities: Turkey, Russia, USA
  • Etymology: Greek diaspeirein, “to scatter.”
  • Definition: Color-change gemstone also sold under the trade name Zultanite.

Alright — continuing from Emerald through the rest of the strict alphabetical list until we finish with Zultanite.

Emerald (Beryl)

  • Category: Precious
  • Group: Beryl
  • Formula: Be₃Al₂(SiO₃)₆ + Cr/V
  • Mohs: 7.5–8
  • Crystal System: Hexagonal
  • Colors: Green to bluish-green
  • RI: 1.57–1.59
  • SG: 2.67–2.78
  • Treatments: Oil/resin filling for clarity
  • Localities: Colombia, Zambia, Brazil, Ethiopia, Afghanistan
  • Etymology: Old French esmeraude, from Latin smaragdus, from Greek smáragdos, “green gem.”
  • Definition: Green beryl prized for vivid color, often with natural inclusions called jardin.

Fluorite

  • Category: Semi-Precious
  • Group: Halide mineral
  • Formula: CaF₂
  • Mohs: 4
  • Crystal System: Isometric
  • Colors: Wide range, often purple, green, yellow, blue
  • RI: 1.433–1.435
  • SG: 3.00–3.25
  • Treatments: Rarely treated
  • Localities: China, Mexico, England, USA
  • Etymology: Latin fluere, “to flow.”
  • Definition: Colorful mineral with perfect cubic cleavage, used in jewelry and as an industrial flux.

Garnet (Group)

  • Category: Semi-Precious
  • Group: Nesosilicates
  • Formula: (X₃Y₂)(SiO₄)₃
  • Mohs: 6.5–7.5
  • Crystal System: Isometric
  • Colors: All colors; red most common
  • RI: ~1.72–1.89
  • SG: ~3.5–4.3
  • Treatments: Generally untreated
  • Localities: Africa, India, Sri Lanka, USA, Russia
  • Etymology: Latin granatus, “pomegranate.”
  • Definition: Diverse silicate group including almandine, pyrope, spessartine, grossular, and andradite.

Grandidierite

  • Category: Semi-Precious
  • Group: Borosilicate
  • Formula: (Mg,Fe)Al₃(BO₃)(SiO₄)O₂
  • Mohs: 7.5
  • Crystal System: Orthorhombic
  • Colors: Bluish-green
  • RI: 1.583–1.639
  • SG: 2.98
  • Treatments: None known
  • Localities: Madagascar, Sri Lanka, Namibia
  • Etymology: Named for French explorer Alfred Grandidier.
  • Definition: Very rare gemstone valued for unique blue-green color.

Hackmanite (Sodalite variety)

  • Category: Semi-Precious
  • Group: Feldspathoid
  • Formula: Na₈Al₆Si₆O₂₄(Cl,S)₂
  • Mohs: 5.5–6
  • Crystal System: Isometric
  • Colors: Violet to pink; tenebrescent color change
  • RI: 1.480–1.490
  • SG: 2.22–2.33
  • Treatments: None known
  • Localities: Afghanistan, Myanmar, Canada
  • Etymology: Named for Finnish geologist Victor Axel Hackman.
  • Definition: Rare sodalite variety with UV-induced reversible color change.

Hematite

  • Category: Semi-Precious
  • Group: Oxide mineral
  • Formula: Fe₂O₃
  • Mohs: 5–6.5
  • Crystal System: Trigonal
  • Colors: Metallic gray to black; red streak
  • RI: 2.940–3.220
  • SG: 5.26
  • Treatments: None
  • Localities: Brazil, Australia, China, USA
  • Etymology: Greek haima, “blood.”
  • Definition: Iron oxide mineral used as gem and ore, recognized by red streak.

Hiddenite (Spodumene)

  • Category: Semi-Precious
  • Group: Pyroxene
  • Formula: LiAlSi₂O₆
  • Mohs: 6.5–7
  • Crystal System: Monoclinic
  • Colors: Green (chromium)
  • RI: 1.660–1.681
  • SG: 3.18
  • Treatments: Rarely treated
  • Localities: USA (North Carolina), Afghanistan, Brazil
  • Etymology: Named for W.E. Hidden.
  • Definition: Rare green spodumene with strong pleochroism.

Iolite (Cordierite)

  • Category: Semi-Precious
  • Group: Cyclosilicate
  • Formula: (Mg,Fe)₂Al₄Si₅O₁₈
  • Mohs: 7–7.5
  • Crystal System: Orthorhombic
  • Colors: Violet-blue, pleochroic
  • RI: 1.542–1.551
  • SG: 2.58–2.66
  • Treatments: None; sometimes heated
  • Localities: India, Sri Lanka, Madagascar, Brazil
  • Etymology: Greek ion, “violet.”
  • Definition: Transparent gemstone historically used for navigation as a polarizing filter.

Jadeite

  • Category: Semi-Precious
  • Group: Pyroxene
  • Formula: NaAlSi₂O₆
  • Mohs: 6.5–7
  • Crystal System: Monoclinic
  • Colors: Green, lavender, white, yellow, black
  • RI: 1.652–1.688
  • SG: 3.30–3.38
  • Treatments: Impregnation, bleaching
  • Localities: Myanmar, Guatemala, Japan
  • Etymology: Spanish piedra de ijada, “flank stone.”
  • Definition: Tough, high-value jade mineral with deep cultural significance.

Jasper

  • Category: Semi-Precious
  • Group: Chalcedony
  • Formula: SiO₂ with impurities
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Variegated patterns
  • RI: ~1.54
  • SG: 2.58–2.91
  • Treatments: Dyeing
  • Localities: Worldwide
  • Etymology: Greek iaspis, “spotted stone.”
  • Definition: Opaque chalcedony with diverse patterns, used decoratively.

Jet

  • Category: Semi-Precious (Organic)
  • Group: Lignite (fossilized wood)
  • Formula: Approx. C₆H₄O
  • Mohs: 2.5–4
  • Crystal System: Amorphous
  • Colors: Black, deep brown
  • RI: 1.66–1.68
  • SG: 1.30–1.35
  • Treatments: Oiling, waxing
  • Localities: England (Whitby), Spain, Turkey
  • Etymology: Greek gagates, “stone of Gages.”
  • Definition: Fossilized wood coal, lightweight and deep black, historically used in mourning jewelry.

Kunzite (Spodumene)

  • Category: Semi-Precious
  • Group: Pyroxene
  • Formula: LiAlSi₂O₆
  • Mohs: 6.5–7
  • Crystal System: Monoclinic
  • Colors: Pink to lilac
  • RI: 1.660–1.676
  • SG: 3.18
  • Treatments: Irradiation, heat
  • Localities: Afghanistan, Brazil, USA
  • Etymology: Named for gemologist George Frederick Kunz.
  • Definition: Pleochroic pink spodumene valued for brilliance.

Kyanite

  • Category: Semi-Precious
  • Group: Aluminosilicate
  • Formula: Al₂SiO₅
  • Mohs: 4.5–5 (lengthwise), 6.5–7 (crosswise)
  • Crystal System: Triclinic
  • Colors: Blue, green, gray, colorless
  • RI: 1.712–1.734
  • SG: 3.53–3.65
  • Treatments: None
  • Localities: Nepal, Brazil, USA, Switzerland
  • Etymology: Greek kyanos, “blue.”
  • Definition: Blue gemstone with directional hardness variation.

Labradorite

  • Category: Semi-Precious
  • Group: Plagioclase feldspar
  • Formula: (Ca,Na)(Al,Si)₄O₈
  • Mohs: 6–6.5
  • Crystal System: Triclinic
  • Colors: Gray to black with labradorescence
  • RI: 1.559–1.573
  • SG: 2.68–2.72
  • Treatments: None
  • Localities: Canada, Finland, Madagascar
  • Etymology: Named for Labrador, Canada.
  • Definition: Feldspar with iridescent color flashes caused by light diffraction.

Lapis Lazuli

  • Category: Semi-Precious
  • Group: Feldspathoid mixture
  • Formula: (Na,Ca)₈(AlSiO₄)₆(S,SO₄,Cl)₁–₂
  • Mohs: ~5–6
  • Crystal System: Isometric (lazurite)
  • Colors: Deep blue with pyrite flecks
  • RI: ~1.50
  • SG: 2.70–2.90
  • Treatments: Dyeing, waxing
  • Localities: Afghanistan, Chile, Russia
  • Etymology: Latin lapis “stone” + Persian lazhward “blue.”
  • Definition: Ancient ornamental stone used in jewelry and pigments.

Larimar (Pectolite)

  • Category: Semi-Precious
  • Group: Inosilicate
  • Formula: NaCa₂Si₃O₈(OH)
  • Mohs: 4.5–5
  • Crystal System: Triclinic
  • Colors: Sky-blue, blue-green, white
  • RI: 1.59–1.63
  • SG: 2.74–2.90
  • Treatments: Usually untreated
  • Localities: Dominican Republic
  • Etymology: Blend of Larissa (discoverer’s daughter) + Spanish mar “sea.”
  • Definition: Rare Caribbean blue pectolite.

Malachite

  • Category: Semi-Precious
  • Group: Carbonate mineral
  • Formula: Cu₂CO₃(OH)₂
  • Mohs: 3.5–4
  • Crystal System: Monoclinic
  • Colors: Rich green with banding
  • RI: 1.655–1.909
  • SG: 3.95–4.00
  • Treatments: Stabilization
  • Localities: DRC, Russia, Namibia, Australia
  • Etymology: Greek malakhē, “mallow.”
  • Definition: Green copper carbonate with distinctive band patterns.

Moldavite

  • Category: Semi-Precious
  • Group: Natural glass (tektite)
  • Formula: SiO₂ + Al₂O₃ + trace elements
  • Mohs: ~5–5.5
  • Crystal System: Amorphous
  • Colors: Bottle-green to olive-green
  • RI: 1.48–1.51
  • SG: 2.32–2.38
  • Treatments: None
  • Localities: Czech Republic
  • Etymology: Named for Moldau (Vltava) River area.
  • Definition: Impact glass from meteorite collision 15 million years ago.

Moonstone

  • Category: Semi-Precious
  • Group: Feldspar
  • Formula: KAlSi₃O₈
  • Mohs: 6–6.5
  • Crystal System: Monoclinic
  • Colors: Colorless, white, gray, peach with adularescence
  • RI: 1.518–1.526
  • SG: 2.56–2.59
  • Treatments: Usually untreated
  • Localities: Sri Lanka, India, Madagascar
  • Etymology: Named for its moon-like sheen.
  • Definition: Feldspar gemstone with internal light play.

Morganite (Beryl)

  • Category: Semi-Precious
  • Group: Beryl
  • Formula: Be₃Al₂(SiO₃)₆ + Mn
  • Mohs: 7.5–8
  • Crystal System: Hexagonal
  • Colors: Pink, peach, salmon
  • RI: 1.583–1.590
  • SG: 2.71–2.90
  • Treatments: Heat to adjust color
  • Localities: Madagascar, Brazil, USA
  • Etymology: Named for financier J.P. Morgan.
  • Definition: Pink beryl valued for delicate pastel shades.

Obsidian

  • Category: Semi-Precious
  • Group: Natural volcanic glass
  • Formula: SiO₂-rich with trace oxides
  • Mohs: ~5–5.5
  • Crystal System: Amorphous
  • Colors: Black, mahogany, snowflake, rainbow varieties
  • RI: 1.48–1.51
  • SG: 2.35–2.60
  • Treatments: None; sometimes dyed
  • Localities: Mexico, USA, Iceland, Japan
  • Etymology: Latin lapis obsidianus, “stone of Obsius,” after a Roman who reportedly discovered it in Ethiopia.
  • Definition: Volcanic glass formed by rapid lava cooling, used for tools, weapons, and jewelry.

Onyx (Banded Chalcedony)

  • Category: Semi-Precious
  • Group: Quartz (cryptocrystalline)
  • Formula: SiO₂
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Parallel bands of black and white; dyed versions in various colors
  • RI: ~1.530
  • SG: 2.58–2.64
  • Treatments: Dyeing to enhance black color
  • Localities: Brazil, Uruguay, India, USA
  • Etymology: Greek onyx, “claw” or “fingernail,” for flesh-toned layers in some varieties.
  • Definition: Layered chalcedony used in cameos and inlay work.

Opal

  • Category: Semi-Precious (mineraloid)
  • Group: Hydrated silica
  • Formula: SiO₂·nH₂O
  • Mohs: 5.5–6.5
  • Crystal System: Amorphous
  • Colors: All hues; precious opal exhibits play-of-color
  • RI: 1.37–1.52
  • SG: 1.98–2.20
  • Treatments: Smoke treatment, impregnation, sugar-acid treatment
  • Localities: Australia, Ethiopia, Mexico, USA
  • Etymology: Sanskrit upala, “precious stone,” via Greek opallios and Latin opalus.
  • Definition: Hydrated silica mineraloid prized for diffraction-based play-of-color.

Pearl (Organic)

  • Category: Semi-Precious (organic gemstone)
  • Group: Nacre from mollusks
  • Formula: CaCO₃ (aragonite) + conchiolin
  • Mohs: 2.5–4.5
  • Crystal System: Orthorhombic (aragonite microcrystals)
  • Colors: White, cream, black, gold, pink, lavender
  • RI: 1.53–1.69
  • SG: 2.60–2.85
  • Treatments: Bleaching, dyeing, luster enhancement
  • Localities: Japan, China, French Polynesia, Australia
  • Etymology: Latin perla, possibly linked to perna, “shellfish.”
  • Definition: Organic gem formed within mollusks, valued for luster and iridescence.

Peridot (Olivine)

  • Category: Semi-Precious
  • Group: Nesosilicate
  • Formula: (Mg,Fe)₂SiO₄
  • Mohs: 6.5–7
  • Crystal System: Orthorhombic
  • Colors: Olive-green to yellow-green
  • RI: 1.65–1.69
  • SG: 3.27–3.37
  • Treatments: Generally untreated
  • Localities: Myanmar, Pakistan, USA (Arizona), China
  • Etymology: Arabic farīdah, “gem.”
  • Definition: Transparent green olivine, prized for fresh, vibrant color.

Pietersite

  • Category: Semi-Precious
  • Group: Quartz (chalcedony aggregate)
  • Formula: SiO₂ with crocidolite inclusions
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Swirling blues, golds, reds; chatoyant
  • RI: ~1.544–1.553
  • SG: 2.58–2.66
  • Treatments: None
  • Localities: Namibia, China
  • Etymology: Named for Sid Pieters, who discovered it in 1962.
  • Definition: Brecciated quartz with fibrous inclusions creating chaotic chatoyancy.

Prehnite

  • Category: Semi-Precious
  • Group: Inosilicate
  • Formula: Ca₂Al(AlSi₃O₁₀)(OH)₂
  • Mohs: 6–6.5
  • Crystal System: Orthorhombic
  • Colors: Pale green, yellow-green, white
  • RI: 1.610–1.650
  • SG: 2.80–2.95
  • Treatments: None
  • Localities: South Africa, Australia, USA
  • Etymology: Named for Colonel Hendrik von Prehn.
  • Definition: Translucent gemstone often forming botryoidal clusters, prized for its gentle green hue.

Pyrite

  • Category: Semi-Precious
  • Group: Sulfide
  • Formula: FeS₂
  • Mohs: 6–6.5
  • Crystal System: Isometric
  • Colors: Brassy yellow (metallic)
  • RI: Opaque
  • SG: 4.95–5.10
  • Treatments: None
  • Localities: Spain, Peru, USA
  • Etymology: Greek pyr, “fire.”
  • Definition: “Fool’s gold” mineral, sometimes cut and polished for jewelry.

Quartz (General)

  • Category: Semi-Precious
  • Group: Silicate
  • Formula: SiO₂
  • Mohs: 7
  • Crystal System: Trigonal
  • Colors: All colors; includes amethyst, citrine, rose quartz, smoky quartz
  • RI: 1.544–1.553
  • SG: 2.65
  • Treatments: Heat, irradiation, dyeing
  • Localities: Worldwide
  • Etymology: German Quarz, origin uncertain.
  • Definition: One of the most abundant minerals, with numerous gem varieties.

Rhodochrosite

  • Category: Semi-Precious
  • Group: Carbonate
  • Formula: MnCO₃
  • Mohs: 3.5–4
  • Crystal System: Trigonal
  • Colors: Pink to red
  • RI: 1.60–1.82
  • SG: 3.45–3.70
  • Treatments: Stabilization for softness
  • Localities: Argentina, Peru, USA (Colorado)
  • Etymology: Greek rhodo, “rose,” + chroma, “color.”
  • Definition: Rose-pink manganese carbonate gemstone, often banded.

Rhodonite

  • Category: Semi-Precious
  • Group: Inosilicate (pyroxenoid)
  • Formula: (Mn,Fe,Mg,Ca)SiO₃
  • Mohs: 5.5–6.5
  • Crystal System: Triclinic
  • Colors: Pink to red with black veins
  • RI: 1.716–1.756
  • SG: 3.40–3.74
  • Treatments: None
  • Localities: Russia, Australia, USA
  • Etymology: Greek rhodo, “rose.”
  • Definition: Pink gemstone often used in cabochons and carvings.

Sard

  • Category: Semi-Precious
  • Group: Chalcedony
  • Formula: SiO₂
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Brownish-red
  • RI: ~1.53
  • SG: 2.58–2.64
  • Treatments: Heat
  • Localities: India, Brazil, Germany
  • Etymology: From Sardis, Asia Minor.
  • Definition: Brownish-red chalcedony used for seals and intaglios.

Sardonyx

  • Category: Semi-Precious
  • Group: Banded chalcedony
  • Formula: SiO₂
  • Mohs: ~7
  • Crystal System: Cryptocrystalline
  • Colors: Red/brown sard with white onyx bands
  • RI: ~1.53
  • SG: 2.58–2.64
  • Treatments: Dyeing
  • Localities: India, Brazil, Germany, USA
  • Etymology: Blend of sard + onyx.
  • Definition: Layered chalcedony used for cameos and carving.

Scapolite

  • Category: Semi-Precious
  • Group: Tectosilicate
  • Formula: (Na,Ca)₄Al₃Si₉O₂₄Cl
  • Mohs: 5–6
  • Crystal System: Tetragonal
  • Colors: Yellow, pink, violet, colorless
  • RI: 1.540–1.579
  • SG: 2.50–2.74
  • Treatments: Heat to improve color
  • Localities: Madagascar, Tanzania, Myanmar
  • Etymology: Greek skapos, “rod.”
  • Definition: Transparent gemstone sometimes displaying cat’s-eye effect.

Sillimanite (Fibrolite)

  • Category: Semi-Precious
  • Group: Aluminosilicate
  • Formula: Al₂SiO₅
  • Mohs: 6.5–7.5
  • Crystal System: Orthorhombic
  • Colors: Brown, green, gray, blue
  • RI: 1.654–1.684
  • SG: 3.23
  • Treatments: None
  • Localities: India, Sri Lanka, Myanmar
  • Etymology: Named for American chemist Benjamin Silliman.
  • Definition: Fibrous aluminosilicate sometimes cut to show a cat’s-eye effect.

Smithsonite

  • Category: Semi-Precious
  • Group: Carbonate mineral
  • Formula: ZnCO₃
  • Mohs: 4–4.5
  • Crystal System: Trigonal
  • Colors: Blue-green, pink, yellow, purple, white
  • RI: 1.619–1.854
  • SG: 4.39–4.45
  • Treatments: Stabilization for softness
  • Localities: Namibia, Mexico, USA
  • Etymology: Named for James Smithson, founder of the Smithsonian Institution.
  • Definition: Zinc carbonate known for its botryoidal crystal habit and pastel colors.

Sodalite

  • Category: Semi-Precious
  • Group: Feldspathoid mineral
  • Formula: Na₈Al₆Si₆O₂₄Cl₂
  • Mohs: 5.5–6
  • Crystal System: Isometric
  • Colors: Royal blue, often with white calcite veining
  • RI: 1.483–1.487
  • SG: 2.27–2.33
  • Treatments: Dyeing to intensify color
  • Localities: Canada, Brazil, Namibia
  • Etymology: Named for sodium content.
  • Definition: Ornamental gemstone sometimes used as a lapis substitute.

Spinel

  • Category: Semi-Precious
  • Group: Oxide mineral
  • Formula: MgAl₂O₄
  • Mohs: 8
  • Crystal System: Isometric
  • Colors: Red, pink, blue, violet, black, colorless
  • RI: 1.712–1.736
  • SG: 3.58–4.06
  • Treatments: Heat; rarely treated otherwise
  • Localities: Myanmar, Sri Lanka, Vietnam, Tanzania
  • Etymology: Latin spinella, “little thorn,” from crystal shape.
  • Definition: Durable gemstone historically mistaken for ruby or sapphire.

Sugilite

  • Category: Semi-Precious
  • Group: Cyclosilicate
  • Formula: KNa₂(Fe,Mn,Al)₂Li₃Si₁₂O₃₀
  • Mohs: 6–6.5
  • Crystal System: Hexagonal
  • Colors: Purple, pinkish-purple, lavender
  • RI: 1.607–1.610
  • SG: 2.74–2.80
  • Treatments: Usually untreated
  • Localities: South Africa, Japan
  • Etymology: Named for Japanese petrologist Ken-ichi Sugi.
  • Definition: Rare purple gemstone often used in cabochons.

Sunstone (Feldspar)

  • Category: Semi-Precious
  • Group: Feldspar (oligoclase or labradorite variety)
  • Formula: (Ca,Na)[Al(Al,Si)Si₂O₈]
  • Mohs: 6–6.5
  • Crystal System: Triclinic
  • Colors: Yellow, orange, red, pink, colorless with aventurescence
  • RI: 1.537–1.547
  • SG: 2.62–2.67
  • Treatments: Rarely treated
  • Localities: USA (Oregon), India, Norway
  • Etymology: Named for glittering appearance reminiscent of sunlight.
  • Definition: Feldspar gemstone showing aventurescence from reflective inclusions.

Tanzanite (Zoisite)

  • Category: Semi-Precious
  • Group: Sorosilicate
  • Formula: Ca₂Al₃(SiO₄)₃(OH)
  • Mohs: 6–7
  • Crystal System: Orthorhombic
  • Colors: Blue, violet, bluish-purple (pleochroic)
  • RI: 1.691–1.700
  • SG: 3.35
  • Treatments: Heat to enhance color
  • Localities: Tanzania (Merelani Hills)
  • Etymology: Trade name by Tiffany & Co., referencing origin.
  • Definition: Rare gemstone noted for vivid blue-violet hues, found only in Tanzania.

Tiger’s Eye (Quartz)

  • Category: Semi-Precious
  • Group: Quartz pseudomorph after crocidolite
  • Formula: SiO₂
  • Mohs: 7
  • Crystal System: Trigonal
  • Colors: Golden to reddish-brown with chatoyancy
  • RI: ~1.544–1.553
  • SG: 2.64–2.71
  • Treatments: Heat to enhance red tones; dyeing
  • Localities: South Africa, Australia, India
  • Etymology: Named for resemblance to tiger’s eye.
  • Definition: Chatoyant quartz with silky luster from parallel fibrous structures.

Topaz

  • Category: Semi-Precious
  • Group: Nesosilicate
  • Formula: Al₂SiO₄(F,OH)₂
  • Mohs: 8
  • Crystal System: Orthorhombic
  • Colors: Colorless, blue, yellow, pink, green, sherry, imperial
  • RI: 1.609–1.643
  • SG: 3.49–3.57
  • Treatments: Irradiation, heat
  • Localities: Brazil, Pakistan, Russia, Sri Lanka
  • Etymology: Greek Topazios, an island in the Red Sea.
  • Definition: Durable gemstone in a wide color range, used in fine jewelry.

Tourmaline (Group)

  • Category: Semi-Precious
  • Group: Borosilicate minerals
  • Formula: (Na,Ca)(Li,Al,Mg)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
  • Mohs: 7–7.5
  • Crystal System: Trigonal
  • Colors: All colors; bicolor and tricolor prized
  • RI: 1.614–1.666
  • SG: 3.02–3.26
  • Treatments: Heat, irradiation
  • Localities: Brazil, Afghanistan, Mozambique, USA
  • Etymology: Sinhalese turmali, “mixed gems.”
  • Definition: Group with exceptional color range; includes elbaite, dravite, schorl.

Turquoise

  • Category: Semi-Precious
  • Group: Hydrated phosphate of copper and aluminum
  • Formula: CuAl₆(PO₄)₄(OH)₈·4H₂O
  • Mohs: 5–6
  • Crystal System: Triclinic
  • Colors: Blue, green, blue-green
  • RI: 1.610–1.650
  • SG: 2.60–2.90
  • Treatments: Stabilization, dyeing, waxing
  • Localities: Iran, USA, Egypt, China
  • Etymology: French turquoise, “Turkish (stone).”
  • Definition: Opaque gemstone valued since antiquity for jewelry and inlay.

Variscite

  • Category: Semi-Precious
  • Group: Hydrated aluminum phosphate
  • Formula: AlPO₄·2H₂O
  • Mohs: 3.5–4.5
  • Crystal System: Orthorhombic
  • Colors: Green, bluish-green
  • RI: 1.563–1.594
  • SG: 2.42–2.58
  • Treatments: Stabilization
  • Localities: USA, Germany, Australia
  • Etymology: From Variscia (Vogtland), Germany.
  • Definition: Rare phosphate mineral often used as a turquoise substitute.

Zircon

  • Category: Semi-Precious
  • Group: Nesosilicate
  • Formula: ZrSiO₄
  • Mohs: 6.5–7.5
  • Crystal System: Tetragonal
  • Colors: Blue, brown, yellow, red, green, colorless
  • RI: 1.777–1.987
  • SG: 3.93–4.73
  • Treatments: Heat to improve or change color
  • Localities: Cambodia, Sri Lanka, Australia
  • Etymology: Persian zargun, “gold-colored.”
  • Definition: High-brilliance natural gemstone, sometimes used as a diamond simulant.

Zultanite (Diaspore)

  • Category: Semi-Precious
  • Group: Oxide hydroxide
  • Formula: AlO(OH)
  • Mohs: 6.5–7
  • Crystal System: Orthorhombic
  • Colors: Color-change green to pink/purple
  • RI: 1.702–1.750
  • SG: 3.39–3.45
  • Treatments: Usually untreated
  • Localities: Turkey
  • Etymology: Trade name honoring Ottoman sultans.
  • Definition: Rare diaspore variety exhibiting strong pleochroism and color change.