Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts. It is typically dark in color (generally dark brown, black or purplish-red), and basaltic or andesitic in composition. Scoria is relatively low in density as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity greater than 1 and sinks in water.

The holes or vesicles form when gases that were dissolved in the magma come out of solution as it erupts, creating bubbles in the molten rock, some of which are frozen in place as the rock cools and solidifies. Scoria may form as part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks and bombs), for instance in Strombolian eruptions that form steep-sided scoria cones. Most scoria is composed of glassy fragments and may contain phenocrysts. Analysis of scoria found in Yemen showed that it was mainly composed of volcanic glass with a few zeolites (e.g., clinoptilolite). The word scoria comes from the Greek σκωρία, skōria, rust. The term cinder is now considered to be synonymous and interchangeable with the term scoria, and scoria is the preferred term used in the scientific literature.

Eruption of scoria frequently form volcanic cones, called either cinder cones or scoria cones. Prior to the 1980s, neither color nor composition was typically a part of the definition and a size range of the particles, e.g. 2 to 24 mm (0.079 to 0.945 in) in diameter, was usually specified. During the 1980s, the size range disappeared from the definition and a requirement that scoria be black or reddish in color, mafic in composition, or combination of both was added.