Marble is a metamorphic rock. Key points:

• Marble forms when limestone (a sedimentary rock, composed mainly of calcite) is subjected to high heat and pressure, causing the calcite crystals to recrystallize into a denser, interlocking texture. This process is called metamorphism.

• The typical composition is carbonate minerals, primarily calcite (CaCO3) or dolomite (CaMg(CO3)2). Pure white marble comes from clean limestone or dolomite, while impurities create the characteristic veins and colors.

• Marble is usually non-foliated (not layered) and has a crystalline texture with a Mohs hardness around 3, making it relatively soft for stone used in sculpture and architecture. Its ability to polish well and its uniform grain contribute to its aesthetic appeal.

• In stonemasonry, the term "marble" can be used more broadly to include rocks that resemble marble or are metamorphosed from limestone, even if not perfectly classified as true marble.

If you’d like, I can pull concise definitions from a specific source or compare marble to limestone and other metamorphic rocks.