Marble is a metamorphic rock, a form of limestone composed mainly of calcium carbonate and found in many countries. It was formed under intense heat and pressure and features veins and swirls of colour. It can be supplied honed or polished to a very high finish and can be found in black, white, gray, green, pink and several other shades. Like all natural stone, marble tiles should be sealed to give them added stain resistance and to aid cleaning. Being acid-sensitive, marble should be cleaned with a neutral ph or slightly alkaline cleaner.