Possible industrial slag or basaltic volcanic glass
Likely iron-rich vitreous slag; natural alternative: basaltic glass (tachylite)
igneous
Category
Industrial Byproduct (Often Mistaken For Obsidian Or Mineral)
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Obsidian
Obsidian (volcanic glass), predominantly amorphous SiO₂
Igneous
Obsidian
Volcanic Glass (SiO2 + impurities)
igneous
Green Quartz (Aventurine)
Silicon Dioxide (SiO2) with inclusions of Fuchsite
mineral
Sodalite
Sodalite (Na8(Al6Si6O24)Cl2)
mineral
Fossilized sea urchin test (likely sand dollar fragment or echinoid fossil)
Echinoidea; calcitic echinoderm skeleton, chiefly CaCO₃
fossil
Black obsidian
Volcanic glass; predominantly rhyolitic silica-rich melt (approximately 70–80% SiO₂), lacking crystalline structure
igneous
Snowflake Obsidian
Obsidian with Cristobalite inclusions
igneous
Polished black tourmaline (likely schorl)
Schorl, NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
mineral
Likely golden mica-bearing metamorphic rock
Micaceous schist, possibly biotite schist; composition commonly quartz + feldspar + biotite/phlogopite mica
metamorphic
Green Aventurine
SiO2 (Silicon Dioxide) with Fuchsite inclusions
mineral
Mahogany Obsidian
Volcanic Glass with Hematite/Magnetite inclusions
igneous
Mixed mineral and rock specimen collection with an obsidian-like centerpiece
Likely black obsidian (volcanic glass), alongside quartz, amethyst, agate/jasper, and other tumbled minerals
igneous
Ironstone Concretion
Ironstone (Limonite/Goethite/Hematite mixture)
sedimentary
Chert (often called Flint in archaeological contexts)
Microcrystalline or cryptocrystalline Silica (SiO2)
sedimentary
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
igneous
Polished black star-shaped stone, likely black obsidian
Obsidian (volcanic glass), primarily SiO₂
igneous
Dalmatian Jasper
Peralkaline rhyolite with Arfvedsonite inclusions
igneous
Possible iron meteorite
Metallic meteorite, likely Fe–Ni alloy (kamacite/taenite)
meteorite
Mother of Pearl (Nacre)
Aragonite (CaCO3) with Organic Matrix
mineral
Obsidian
Volcanic Glass (Amorphous SiO2)
igneous
Sandstone Concretion
Sedimentary concretion
Sedimentary
Mica-rich Sandstone or Micaceous Schist Weathering
Micaceous Schist (Muscovite-Quartz composition)
Metamorphic or Sedimentary depending on local grade
Limonite Concretion
Limonite (Hydrated iron(III) oxide-hydroxides, FeO(OH)·nH2O)
sedimentary
Quartzite
Quartzite (primarily SiO2)
metamorphic
Serpentine
Antigorite/Lizardite/Chrysotile group, Mg3Si2O5(OH)4
metamorphic
Concretion in Shale
Siltstone/Shale with Siderite or Calcareous concretions
sedimentary
Quartzite
Quartzite (primarily SiO2)
metamorphic
Milky Quartz
Quartz (Silicon Dioxide, SiO2)
mineral
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurity)
igneous
Obsidian (Gold Sheen)
Obsidian (SiO2 + impurities)
igneous
Mica Schist with Pyrite
Muscovite/Biotite Schist with Iron Sulfide (FeS2)
metamorphic
Black Obsidian
Volcanic Glass (predominantly SiO2)
igneous
Mahogany Obsidian
Volcanic glass containing Hematite or Magnetite (SiO2 + Fe2O3)
igneous
Siderite Concretion
Ironstone (Siderite) Concretion, FeCO3
sedimentary
Sandstone Concretion
Sandstone (Sedimentary rock, mostly SiO2)
sedimentary
Snowflake Obsidian
Volcanic glass with Cristobalite inclusions (70–75% SiO2)
Igneous
Sodalite
Sodalite (Na8(Al6Si6O24)Cl2)
mineral
Ironstone Concretion
Ferruginous Siltstone/Ironstone (Fe2O3)
sedimentary
Aura Quartz (Pink/Rose Aura Quartz)
Silicon Dioxide (SiO2) bonded with Platinum and/or Silver
mineral
Carborundum
Silicon Carbide (SiC)
mineral
Obsidian
Obsidian (principally SiO2)
igneous