A metamorphic rock formed by alteration of limestone or dolomite, often irregularly colored by impurities, and used especially in architecture and sculpture. A piece of this rock. A sculpture made from this rock. Something resembling or suggesting metamorphic rock, as in being very hard, smooth, or cold: a heart of marble; a brow of marble. Games A small hard ball, usually of glass, used in children's games. Games Any of various games played with marbles. Slang Common sense; sanity: completely lost his marbles after the stock market crash. Marbling. To mottle and streak (paper, for example) with colors and veins in imitation of marble. Composed of metamorphic rock: a marble hearth. Resembling metamorphic rock in consistency, texture, venation, color, or coldness.