Sedimentary Rock (Specifically A Concretion Or Possibly A Geode)

Weathered sandstone

Quartz-rich clastic sandstone, likely ferruginous arkose or litharenite

Geode

Quartz nodule / Chalcedony-lined cavity

Geode

Silicon Dioxide (Quartz/Chalcedony) in carbonate or volcanic host rock

Limonite Concretion

Goethite/Limonite (FeO(OH)·nH2O)

Iron Concretion

Ferruginous concretion (Fe2O3·nH2O / FeO(OH))

Quartz Geode

Crystalline Silicon Dioxide (SiO2)

Iron-stained quartz or chert fragment

Quartz (SiO₂), possibly microcrystalline quartz/chert with limonite staining

Amethyst (Chevron or Banded variety)

Silicon Dioxide with Iron impurities (SiO2)

Basalt

Aphanitic Mafic Igneous Rock

Druzy Quartz (part of a Geode)

Silicon Dioxide (SiO2)

Quartz geode

Microcrystalline and crystalline quartz (SiO₂), possibly with iron-oxide staining and minor calcite

Sandstone Concretion

Sandstone (Sedimentary Rock) with silica or calcite cement

Conglomerate (possibly concrete-like natural breccia)

Lithified, poorly sorted clastic rock with angular to subrounded mineral and rock fragments in a finer matrix

Quartz geode

Silica geode; predominantly microcrystalline and crystalline quartz (SiO₂), possibly with chalcedony and minor calcite

Likely weathered pyrite nodule

Pyrite (FeS₂), possibly a small marcasite/pyrite concretion

Botryoidal chalcedony (possibly agate/geode material)

Microcrystalline quartz, SiO₂; commonly intergrown with calcite and iron oxides

Sandstone Concretion

Ferruginous Sandstone Concretion

Quartz Geode

Silicon Dioxide (SiO2)

Limonite Concretion

Limonite (Hydrated iron oxide) FeO(OH)·nH2O

Goethite Concretion

Goethite (FeO(OH))

Limonite Concretion

Limonite (FeO(OH)·nH2O) Pseudomorph

Limonite Concretion

Limonite (Hydrated Iron Oxide) FeO(OH)·nH2O

Agate Geode

Silicon Dioxide (SiO2)

Likely ironstone concretion

Iron-oxide-cemented sedimentary concretion, commonly hematite/goethite-rich

Likely weathered sandstone or iron-stained sedimentary pebble

Indeterminate clastic sedimentary rock; possibly quartz-rich sandstone

Quartz Geode

Silicon Dioxide (SiO2) in a sedimentary or volcanic host

Quartz Druzy (Geode Fragment)

Silicon Dioxide (SiO2)

Quartz Geode (Amethyst / Clear Quartz)

Silicon Dioxide (SiO2)

Gneiss (specifically a weathered or stream-worn specimen)

Gneiss (Foliated Metamorphic Rock)

Gravel or Pebble

Clastic Sedimentary Grain (typically composed of Quartz or Silicate minerals)

Flint (Chert Nodules)

SiO2 (Cryptocrystalline Quartz)

Amethyst Lace Agate / Banded Amethyst

Silicon Dioxide (SiO2) with Fe3+ impurities

Quartz Geode

Silicon Dioxide (SiO2)

Sandstone Concretion

Clastic Sedimentary Rock (primarily SiO2 with mineral cement)

Quartz geode

Quartz (SiO₂), likely in a chalcedony or other silica-rich host rind

Quartz Geode

Crystalline Quartz (SiO2) within a sedimentary or volcanic host

Geode (Unopened Keokuk Geode)

Silicon Dioxide (SiO2) - Chalcedony / Quartz exterior

Quartz Geode

Silicon Dioxide (SiO2)

Geode

Quartz / Chalcedony nodule

Iron-rich weathered rock, possibly limonite/goethite-bearing laterite

Limonite (amorphous hydrated iron oxides), commonly mixed with goethite (α-FeO(OH)) and hematite

Tumbled quartz-rich stone, possibly druzy quartz or chalcedony

Quartz/chalcedony (SiO₂); exact identification is uncertain from the photograph

Quartz Geode

Silicon Dioxide (SiO2) in Geode form