Marble is a non-foliated calcium carbonate metamorphic rock. It is formed by limestone or dolostone (dolomitic marble) being subjected to the heat and pressure conditions associated with contact or regional metamorphism. This commonly occurs at convergent plate boundary settings. Both limestone and marble comprise primarily of calcite, but the characteristic differences between the two are their grain sizes and textures. Increasing metamorphic intensity recrystallizes the calcite into coarser interlocking crystals, creating a sugary equigranular texture, as seen in the image. Apart from its granular texture, its characteristic calcite composition makes marble soft and reactive to hydrochloric acid. Similarly, a dolomitic marble would react to acid but less vigorously and only when powdered due to its dolomite protolith. Pure marble is white in colour but often contains impurities and secondary minerals, such as iron oxides or micas from recrystallized clays, producing colourless to pale shades of pink or yellow to gray and black.