Marble is a metamorphic rock formed when limestone is transformed under extreme pressure. Pure marble is a white calcite crystal, but most marbles are enriched by impurities which give it special colors and patterns. It is softer and more brittle than granite, but as a cladding stone it is considered a luxury material prized for its distinctive mottled look. It is usually polished but can also be used in rough form.

Other polished stones, including travertine, onyx, and serpentine, are sometimes referred to as marble. Although scientifically incorrect, this classification is accepted for some commercial uses. On this website the scientific definition is used and other materials are classified accordingly.

A polished finish is common but does not define marble here. Rough marble remains included, while polishing travertine or serpentinite does not turn those stones into marble.