Scoria

Scoria is a dark, highly vesicular volcanic rock riddled with gas bubble holes (vesicles). It is typically basaltic to andesitic in composition and ranges in color from black and dark grey to reddish-brown when oxidized.

Unlike pumice, scoria is denser and usually sinks in water, with thicker bubble walls and a more crystalline groundmass. It often forms the fragments thrown out during eruptions, accumulating as cinder cones.

Widely sold as "lava rock," it is a familiar landscaping and barbecue material.

Scoria forms when gas-charged mafic to intermediate lava is erupted explosively and frothing magma is blown into the air. As the lava fragments fly out and cool quickly, dissolved gases escape, leaving abundant vesicles frozen into the solidifying rock.

These fragments pile up around vents to build cinder (scoria) cones. The reddish color of some scoria comes from oxidation of iron during or after eruption. It is common in basaltic volcanic fields worldwide, including Hawaii, Iceland, and the western United States.

Identify scoria by its dark color (black, grey, or red-brown), rough surface, and abundant gas vesicles, with a hardness around 5-6. It is lightweight but denser than pumice and usually sinks in water.

Compared with pumice, scoria is darker, heavier, and has thicker bubble walls; pumice is pale, frothy, and floats. It differs from solid basalt by its highly porous, bubbly texture.

The combination of dark color and a heavy, holey, cinder-like body is diagnostic.

Scoria is widely used as landscaping "lava rock" for mulch, ground cover, and decorative beds, and as drainage material. Its porous, heat-resistant nature makes it popular for gas barbecue grills and fire pits.

It is also used as lightweight aggregate in concrete and building blocks, as road surfacing, and in water filtration and hydroponics.

Scoria has little jewelry value but is an economically important construction and horticultural material.