Organic (Geological Interest)

Ocellar Rhyolite

Rhyolite (featuring orbicular/ocellar quartz and feldspar textures)

Xenolith in Basalt

Mafic Igneous Rock with Felsic Inclusion

Sandstone Concretion

Septarian Concretion (Nodule)

Mookaite Jasper

Radiolarite (Silicified radiolarian chert), SiO2

Tuff with Quartz and Calcite Veining

Lithic Tuff (Silicified)

Chert Conglomerate (Potomac Puddle Stone)

Sedimentary Conglomerate with Iron-Oxide matrix

Shale with Quartz Veins

Fissile Sedimentary Rock (SiO2)

Basalt (River Pebble)

Basalt

Sandstone (River Pebble)

Clastic Sedimentary Rock (primarily Quartz)

Conglomerate

Conglomerate (clastic sedimentary rock)

Quartzite with Quartz Veining

Quartzite (primarily SiO2)

Basalt with Weathered Crust

Aphanitic Mafic Igneous Rock (Basalt)

Coquina

Biogenic Limestone

Quartzite (Quartz-rich Cobble)

Quartzite (SiO2)

Chert (with Quartz veins)

Chert (Microcrystalline Silica, SiO2)

Volcanic Tuff (Breccia)

Pyroclastic Tuffaceous Rock

Weathered quartz-rich pebble

Likely granitoid (quartz and feldspar), but the photo does not support a precise identification

Ferruginous Sandstone

Clastic Sedimentary Rock (Iron-rich)

Likely small iron-rich concretion or weathered mineral fragment

Indeterminate; possibly limonite/goethite-bearing material with quartz or feldspar

Lava Rock with Fabric Impression

Scoria or Basalt with anthropogenic impressions

Greenstone / Basalt

Aphanitic Mafic Igneous Rock (likely altered Basalt)

Chert (Pebble)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Graywacke

Argillaceous sandstone

Gossan

Iron-rich regolith (mainly Limonite and Goethite, FeO(OH)·nH2O)

Greywacke with Quartz Veins

Lithic Wacke (SiO2 enriched)

Quartz-veined Metamorphic Rock

Quartz Vein in Phyllite or Schist (SiO2 in Silicate Matrix)

Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)

Mafic volcanic rock (formula primarily (Mg, Fe)2SiO4 and CaAl2Si2O8)

Tiny rounded brown mineral grain

Indeterminate; possibly iron oxide, garnet, or a weathered sulfide grain

Unidentified pink-and-green mineral-bearing rock

Not reliably identifiable from the image; possible pink feldspar with green epidote or another green mineral

Iron-stained quartz-rich rock (possibly ironstone or ochre-coated quartz)

Undetermined ferruginous rock; yellow-brown coloration likely from goethite and/or limonite (hydrated iron oxides)

Iron-rich sandstone (ferruginous sandstone)

Quartz-rich sandstone cemented or stained by iron oxides, commonly hematite (Fe₂O₃) and goethite (FeO(OH))

Olivine-bearing peridotite (likely a small mantle-rock fragment)

Peridotite, chiefly olivine ((Mg,Fe)₂SiO₄), with possible pyroxene

Quartz vein in host rock

Quartz (SiO₂), likely milky quartz; host rock is uncertain from the photo

Limestone with Calcite Crust

Sedimentary Calcium Carbonate (CaCO3)

Granite boulder

Granitic rock, chiefly quartz, feldspar, and mica

Limonitic Sandstone with Inclusion

Limonitic Quartz Sandstone (primarily SiO2 with FeO(OH)·nH2O)

Biotite Schist

Biotite Gneiss/Schist (approximate formula K(Mg,Fe)3AlSi3O10(OH)2)

Ironstone Concretion (Ironstone Gourd)

Limonite/Goethite Concretion (FeOO(H))

Iron-rich rock fragment, possibly limonite/goethite

Limonite (mixture of hydrated iron oxides, commonly goethite, FeO(OH))

Weathered rock with a tan mineral vein or inclusion

Indeterminate from the photograph; possibly a quartz- or feldspar-rich vein in dark metamorphic host rock

Green Schist

Greenschist (Chlorite Schist)

Epidote and Quartz in Basalt

Epidote [Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)] in Metabasalt