Marble is a metamorphic rock, which means 'rock that has changed'. Metamorphic rocks are formed from other materials already existing on the earth's surface, primarily composed of calcite or dolomite minerals. Calcite is a form of calcium carbonate (CaCO3), and it commonly occurs as crystals within the marble matrix.
During the metamorphic process that forms marble, the original sedimentary rocks, such as limestone or dolomite, undergo intense heat and pressure. These conditions cause the minerals within the rock to recrystallize and rearrange, giving rise to the characteristic interlocking calcite crystals seen in marble.
The size, shape, and arrangement of the crystals can vary when marble forms, depending on factors such as the duration and intensity of the metamorphic process. In some cases, the crystals may be large and easily visible, giving the marble a distinctive crystalline appearance. In other instances, the crystals may be smaller and densely packed, resulting in a smoother texture.
The presence of calcite crystals in marble contributes to its aesthetic appeal and gives it a unique pattern and colouration. The crystals can reflect light, producing a characteristic lustre and enhancing the visual appeal of the stone.
Marble can contain mineral impurities in addition to its primary calcite or dolomite mineral composition. These impurities can introduce various colours and patterns to the marble, resulting in a wide range of aesthetically pleasing variations. Some common mineral impurities found in marble include clay minerals, iron oxides, micas, graphite, pyrite, and quartz.
It's important to note that the presence and type of mineral impurities in marble can vary depending on the specific geological conditions and location where the marble was formed. As a result, different types of marble can exhibit different impurities, resulting in a wide range of colours, patterns, and characteristics.
Igneous rocks are formed directly from lava or the earth's molten core. Marble is formed from limestone. In geological terms, Marble is a crystallized form of limestone or dolomite.
Stonecutters use a broader definition – we'll call the crystallized forms of a few other stones Marble, too, as they have similar veining and take the same high gloss polish in a way other stones don't. This is because stone cutters have been working with Marble for thousands of years, long before anyone could tell the difference between calcium carbonate and magnesium silicate.