Igneous Rock (Volcanic Glass)

Obsidian

Obsidian (Volcanic Glass, primarily SiO2)

Polished dark stone, likely basalt or gabbro

Basalt (fine-grained mafic rock), possibly gabbro; chiefly plagioclase and pyroxene with iron-magnesium minerals

Vesicular volcanic rock, likely pumice

Pumice (highly vesicular volcanic glass; composition commonly rhyolitic to andesitic)

Polished black stone, likely obsidian or dark basalt

Volcanic glass (obsidian), possibly basalt; exact identification requires testing

Likely raw mica-bearing mineral fragment

Possibly biotite mica or a biotite-rich metamorphic fragment; identification is limited by the dark, blurred image

Pumice

Pumice (a highly vesicular extrusive igneous rock)

Scoria (vesicular basalt)

Vesicular basalt; chiefly plagioclase, pyroxene, and iron-rich glass/oxides

Tumbled purple jasper (likely rhyolite-derived jasper)

Microcrystalline quartz (SiO₂) with iron/manganese oxide impurities; possibly purple rhyolitic jasper

Basalt

Extrusive mafic igneous rock

Black beach pebble (likely basalt)

Basalt, a fine-grained volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine

Snowflake Obsidian

Volcanic glass with Cristobalite inclusions (70–75% SiO2)

Vesicular volcanic rock, likely pumice or scoriaceous rhyolite

Highly vesicular felsic volcanic rock; commonly rhyolitic pumice (composition dominated by volcanic glass, quartz, and alkali feldspar)

Basalt

Basalt (composition: Plagioclase, Pyroxene, and Olivine)

Small rounded pebble, likely basalt

Fine-grained mafic volcanic rock (likely basalt); image alone cannot confirm composition

Granite and Basalt Specimen

Plutonic and Volcanic Igneous Rocks

Basalt

Extrusive Igneous Rock

River Cobble (likely Metasedimentary or Basalt)

Metamorphosed Siltstone or Altered Fine-grained Igneous Rock

Rounded dark beach pebble, likely basalt

Fine-grained basalt, possibly andesite or dark chert

Pumice (likely)

Vesicular volcanic rock, typically rhyolitic to dacitic glass with minor feldspar and other minerals

Dark Agate / Chalcedony

Microcrystalline Quartz (SiO2)

Black volcanic rock (possibly obsidian or basalt)

Indeterminate from the image; candidates include volcanic glass (obsidian, SiO₂-rich) or basalt (plagioclase–pyroxene-rich)

Pumice

Pumice (Vesicular intermediate to felsic volcanic glass)

Snowflake Obsidian

Volcanic Glass with Spherulite inclusions (Amorphous SiO2 + Cristobalite)

Porphyritic basaltic-andesite cobble

Porphyritic andesite to basaltic andesite; plagioclase-rich volcanic rock

Amygdaloidal Rhyolite

Rhyolite (felsic volcanic rock) with amygdules

Dark volcanic sand and basalt fragment

Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine

Agate

Silicon dioxide (chalcedony), SiO2

Agate

Silicon dioxide (Chalcedony), SiO2

Agate

Silicon dioxide (SiO2)

Agate

Silicon Dioxide (Chalcedony), SiO2

Lava Stone

Basalt / Volcanic Rock

Agate

Silicon Dioxide (Chalcedony), SiO2

Quartz Vein in Basalt

Silicon Dioxide (SiO2) in Mafic Igneous Rock

Basalt (probable)

Basalt; chiefly plagioclase feldspar, pyroxene, and minor olivine or magnetite

Scoria

Extrusive Igneous Rock

Likely weathered basalt or andesite fragment

Fine-grained volcanic rock, likely basaltic-andesite; exact identification requires testing

Obsidian

Volcanic glass (SiO2 - Silicon Dioxide with impurities)

Dark mafic rock, likely basalt

Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine or iron oxides

Weathered basaltic rock

Basalt, chiefly plagioclase feldspar, clinopyroxene, and minor olivine or iron oxides

Dark gray weathered pebble, likely basalt

Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides

Pyrite-bearing Rock

Iron Sulfide (FeS2)

Mahogany Obsidian

Obsidian (Volcanic Glass with Hematite/Magnetite inclusions)