Obsidian
Volcanic Glass (SiO2)
igneous
Category
Non Geological
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Green Opal
Opal (Hydrated silica, SiO2·nH2O)
mineral
Quartzite pebble
Quartzite (SiO2 dominated)
metamorphic
Anthracite Coal
Anthracite (Carbon-based sedimentary-metamorphic rock)
metamorphic
Sea Glass (often mistaken for Gemstones)
Amorphous Silica (SiO2) with trace additives
mineraloid
Chert (Pebble)
Silicon Dioxide (SiO2)
sedimentary
Pink Marble
Recrystallized Limestone (Calcium Carbonate, CaCO3)
metamorphic
Bismuth Crystal
Bismuth (Bi)
mineral
Quartzite
Metamorphosed Quartz Sandstone (SiO2)
metamorphic
Quartzite
Metamorphosed Quartz Sandstone
metamorphic
Quartzite Pebble
Quartzite (SiO2)
metamorphic
Quartzite
Quartzite (primarily SiO2)
metamorphic
Quartzite River Rock
Quartzite (primarily SiO2)
metamorphic
River Stone (Quartzite)
Metamorphic Quartzite (primarily SiO2)
metamorphic
Quartzite River Pebble
Metamorphosed sandstone (SiO2)
metamorphic
Quartzite with Chlorite staining
Metamorphic Quartzite (SiO2 with Chlorite inclusions)
metamorphic
Amber
Succinite (approximate formula C10H16O)
mineraloid
Quartzite Cobble
Quartzite (SiO2)
metamorphic
Quartzite (with Iron Staining)
Metamorphosed Quartz Sandstone (SiO2)
metamorphic
Quartzite pebble
Quartzite (SiO2)
metamorphic
Amber
Succinite (C10H16O)
mineral
Quartzite Pebble
Quartzite (primarily SiO2)
metamorphic
Quartzite
Quartzite (primarily SiO2)
metamorphic
Boulder Opal
Amorphous Hydrous Silica (SiO2·nH2O)
mineral
Amber
Succinates (Organic Gemstone)
mineral
Quartzite River Rock
Metaquartzite (majorly SiO2)
metamorphic
Snowflake Obsidian
Volcanic glass with Spherulite inclusions (SiO2)
igneous
Obsidian
Obsidian (Volcanic Glass)
igneous
Quartzite with Hematite
Metamorphosed Quartz Sandstone (SiO2 with Fe2O3 inclusions)
metamorphic
Polished Unakite (likely)
Unakite: quartz + epidote + pink K-feldspar, commonly with minor chlorite
metamorphic
Quartzite
Metamorphic Quartz Arenite
metamorphic
Quartzite
Metamorphic Quartzite (SiO2)
metamorphic
Quartzite
Metamorphic Quartzite (SiO2-rich rock)
metamorphic
Opalite Egg
Synthetic Glass (Simulated Opal)
mineral
Chert Cobble / Quartzite River Stone
Microcrystalline Quartz (SiO2) / Quartzite
sedimentary
Black Obsidian
Volcanic Glass (Amorphous Silicon Dioxide, SiO2)
igneous
Black Obsidian cabochon
Volcanic Glass (SiO2 + impurities)
igneous
Quartzite Pebble
Metamorphic Quartzite (SiO2)
metamorphic
Basalt (River Pebble)
Basalt
igneous
Opalite
Man-made synthetic glass with opalescent effects (SiO2 + metal oxides)
mineraloid
Mahogany Obsidian
Volcanic Glass with Hematite inclusions (SiO2 + Fe2O3)
igneous
Obsidian
Volcanic Glass (70-75% SiO2)
igneous