Volcanic Glass (Igneous)

Snowflake Obsidian

Obsidian with Cristobalite inclusions (70–75% SiO2)

Polished black stone, likely obsidian

Obsidian (volcanic glass; mostly silica-rich rhyolitic composition)

Weathered porphyritic volcanic rock (likely rhyolite or andesite)

Porphyritic rhyolite–andesite; chiefly quartz/feldspar with iron-oxide-stained groundmass

Tumbled blue-gray stone, likely basalt or slate

Probable fine-grained basalt; exact identification is uncertain from the photograph

Porphyritic Rhyolite (Red)

Rhyolite (Porphyry)

Scoria

Vesicular basaltic scoria; chiefly plagioclase, pyroxene, olivine, and volcanic glass

Obsidian

Volcanic Glass (Amorphous Silicon Dioxide)

Black volcanic rock, possibly basalt or obsidian

Likely basalt (fine-grained mafic silicate rock); obsidian is a possible alternative

Pumice

Vesicular Rhyolite / Pumice

Snowflake Obsidian

Volcanic Glass with Spherulite inclusions (SiO2)

Pumice

Pumice (Volcanic Rock / Glass Froth)

Snowflake Obsidian

Volcanic glass containing Spherulites (SiO2)

Clear Quartz

Silicon Dioxide (SiO2)

Mahogany Obsidian

Volcanic Glass with Hematite/Magnetite inclusions (SiO2 + Fe2O3)

Black Obsidian

Volcanic Glass (Amorphous Silicon Dioxide)

Green Sheen Obsidian

Volcanic Glass (SiO2 with microscopic inclusions)

Mahogany Obsidian

Volcanic glass containing Magnetite/Hematite

Obsidian

Volcanic glass (high silica SiO2)

Pumice

Pumice (a highly vesicular volcanic glass)

Pumice

Pumice (Vesicular Felsic Igneous Rock)

Snowflake Obsidian

Volcanic glass (SiO2) with Cristobalite (SiO2) inclusions

Polished black pebble, likely basalt

Basalt; predominantly plagioclase feldspar, pyroxene, and commonly olivine or magnetite

Scoria (vesicular volcanic rock)

Basaltic to andesitic scoria; chiefly plagioclase, pyroxene, and volcanic glass

Vesicular basalt (possibly scoriaceous basalt)

Basalt; predominantly calcium-rich plagioclase, clinopyroxene, and often olivine, with volcanic glass

Pumice

Pumice (Volcanic Rock / Glass Foam)

Pumice

Pumice (Volcanic Rock / Glass Froth)

Snowflake Obsidian

Volcanic Glass with Cristobalite Inclusions (SiO2)

Pumice

Vesicular volcanic glass (primarily SiO2)

Obsidian (likely volcanic glass)

Obsidian; predominantly rhyolitic glass, chiefly SiO₂

Likely polished/tumbled dark mineral or glassy rock fragment

Indeterminate from image; possibly hornblende, schorl tourmaline, or obsidian

Agate-bearing rhyolite (likely polished/cut specimen)

Silica-rich volcanic rock containing microcrystalline quartz/chalcedony, SiO₂, with feldspar and possible iron oxides

Black glassy volcanic rock, likely scoria or obsidian

Vesicular basaltic scoria (probable); possibly dark obsidian

Tumbled black stone, likely obsidian

Obsidian (volcanic glass; predominantly SiO₂)

Polished black stone, likely black obsidian

Obsidian (volcanic glass), chiefly SiO₂

Pumice

Volcanic pumice; chiefly vesicular volcanic glass composed of rhyolitic to dacitic silicates

Pumice (likely)

Vesicular volcanic glass, typically rhyolitic or dacitic in composition

Mahogany Obsidian

Volcanic Glass (predominantly SiO2 with Hematite/Magnetite inclusions)

Obsidian (likely black obsidian)

Volcanic glass; predominantly rhyolitic silica glass (SiO₂-rich)

Polished mahogany obsidian

Volcanic glass; rhyolitic obsidian, commonly with iron-oxide coloration

Red scoria (vesicular basalt)

Scoriaceous basalt; chiefly plagioclase, pyroxene, and iron-rich volcanic glass

Lace Agate (specifically Red Lace Agate)

Silicon Dioxide (SiO2) - variety of Chalcedony/Quartz

Obsidian

Volcanic glass (silica-rich rhyolitic melt; primarily SiO₂)