More than one type

“Lava rock” – a colloquial, not formal rock name. In collections and bead trays it usually means vesicular basalt (scoria) – basaltic lava full of gas bubbles. In landscaping and construction, the same material is often called cinder or scoria.

Related volcanic textures

Pumice – a frothy, silica-rich “cousin” that can even float. Obsidian – volcanic glass with few or no bubbles. All are volcanic but differ in chemistry and gas content.

Gases dissolved in magma

Dissolved H₂O, CO₂, SO₂, etc. in magma begin to exsolve near the surface as pressure decreases – forming bubbles, like opening a carbonated drink.

“Frozen bubbling”

In liquid basaltic lava, bubbles rise and expand as the melt cools. If the lava solidifies quickly, the bubbles are trapped as vesicles. Where bubbles burst, the walls collapse leaving jagged holes and thin partitions.

After the eruption

Later, mineral-rich solutions can fill the bubbles with secondary minerals – calcite, zeolites, quartz, prehnite, or chlorite. Such filled bubbles are called amygdales, and the rock is an amygdaloidal basalt.

Palette and surfaces

• Black to anthracite — most characteristic of basaltic scoria.
• Gray — quenched surfaces and silica-rich lavas.
• Reddish brown — iron oxidation on air-exposed ash ("cinder") cones.

Vesicle size – from pinprick to marble. Walls often sharp, angular; drum-polished beads feel pleasantly matte.

Flow textures (bonus)

• Pāhoehoe — smooth, ropy surfaces; frequent abundance of vesicles at flow tops.
• ‘A‘ā — rough, clinkery flows; dominated by scoria fragments.
• Spatter lava / bombs — drop-shaped pieces with elongated vesicles.

Photography tip: Side light at ~30° angle casts fine shadows from vesicle edges and beautifully reveals the "foam" architecture.

Bubbles and amygdales

Look for round to irregular cavities with glassy, crystalline, or earthy coatings. If filled, they may contain calcite, zeolites, chalcedony, prehnite, or quartz, forming tiny geodes.

Phenocrysts and microlites

Fine, sharp plagioclase (white), pyroxene (dark), or olivine (greenish) crystals may sit in a fine matrix. Microlites often orient with flow, forming subtle streaks.

Oxidation edges

Rusty red halos around vesicle edges indicate iron oxidation – a common phenomenon in scoria exposed to air and steam during eruption.

Global stage

Where volcanoes breathe: Iceland, Hawaii, Canary Islands, Italy (Etna, Stromboli), Eifel (Germany), East African Rift and many volcanic arcs and fields worldwide.

Classic cones

Ash cones such as Parícutin (Mexico), and fields around Flagstaff, Arizona, are typical scoria “factories” – thousands of meters of ash and lapilli with vesicular fragments.

Slag (industrial)

More glassy, often with metallic luster or elongated vesicles; may have swirls and human-made flow textures. Environment is important (near foundries/railways).

Dyed porous stones

Some porous limestones or ceramic beads are dyed black to imitate lava. Under magnification, paint accumulations in pores and an overly “painted” appearance are visible.

Obsidian

Volcanic glass without vesicles (except pumice variants). Breaks conchoidally, has a high glassy luster—a very different feel in the hand.

Scoria andesite

Intermediate composition vesicular lava can be similar but is more often grayish and with more plagioclase phenocrysts. Chemistry confirms the difference.

Quick check

• Abundant, round to jagged vesicles.
• Dark mafic palette; often weakly magnetic.
• Matte feel on worn pieces; freshly broken – grainy.

Fun magnet test

A small magnet can easily attract due to magnetite. It won't "stick" like iron, but you'll often feel a slight pull.

The reality of porosity

• Bubbles are easy to "clog" with dust and liquids; a soft brush and mild soap help.
• Rinse well and dry; avoid long soaking to prevent residue buildup.

Heat and shock

• Basalt handles heat better than many rocks, but sudden temperature shocks can chip edges.
• When hot, let cool gradually – this preserves microcracks.

Surface and storage

• Matte surfaces can absorb oils; wipe with a clean dry cloth.
• Keep separate from very hard minerals to keep edges sharp.

Everyday materials

Vesicular basalts and scoria are used as lightweight filler, landscaping stone, grill stones, and drainage material. Low density and rough surface are useful wherever porosity helps.

Beads and handicrafts

"Lava stone" beads are usually – worn scoria. The pores provide a tactile, matte aesthetic. Under a magnifier, you'll see real volcanic bubbles, not machined pits—each bead is essentially a tiny piece of eruption history.