Marble is a metamorphic rock, formed by the low-to-high grade metamorphism of limestone, limestone breccias, conglomerates, and sometimes also dolomite rock. Metamorphism is caused by the heat and pressure which occur when continental plates collide (regional metamorphism) to create mountains or when the intrusion of igneous rocks causes an increase in temperature (contact metamorphism). The recrystallisation that takes place during this metamorphism causes the grains within the rock to interlock making in more durable and able to take a polish. Minerals, or impurities, within a rock will affect its colour while the different forces that it is exposed to during metamorphism will affect its composition. Pressure can contort rocks (folding), it can cause the minerals within a metamorphic rock to align along one plane (foliation), which then allows them to be split, or it can stretch and break up clasts or individual grains within a rock (shearing). Marble is popular for sculpture because it is hard and usually fine-grained enough to hold detail without being as hard as granite and can also take a polish. Numerous sources of marble were exploited in the Roman period with the most favoured white marbles coming from Luna (modern Carrara) in northern Italy, various locations around the Aegean and Sea of Marmara, as well as Aphrodisias and Dokimeion in inland Turkey.