True marble was originally limestone that has been altered by heat and pressure over time. These materials originally deposited as unconsolidated sediments under water by chemical precipitation and by living organisms, have been buried, heated and subject to huge confining pressures over lengthy periods of time. Such processes of metamorphism have transformed the limestone to a denser, harder crystalline rock, we now call Marble.
Relic features of the original limestone can be preserved although much altered. Furthermore selective partial melting can result in migration of liquids and other volatiles through the fabric of the marble further altering its composition and structure giving rise to some stunning features including:
To complicate things further there are other highly decorative stones whose origins are not limestone but are colourful, highly figured and that also can be polished and are therefore known by masons as marble.
Tradition maintains that even today several well known materials continue to be called marbles when in fact they are technically dense limestones that will take a high polish. Example: Perlato Sicilia