Although both are stones and both are quarried from the earth, granite and marble (and marble’s relatives – limestone, onyx and travertine) are very different from each other. Granite is formed deep in the earth’s mantle at extremely high temperatures, and is a very hard, resistant stone made of crystallized minerals.
The marble family – limestone, travertine, marble, onyx – starts out as sediment – animal skeletons and shells, plant matter, silt – at the bottom of bodies of water. After millions of years this solidifies into stone. Because its main component is calcium, it can be affected by acids such as vinegar and citrus beverages.
The term “granite” is used to cover a group of related stones, all of which have their origin deep in the earth’s molten mantle. As this extremely hot liquid material rises and cools, it forms a crystalline, granular structure, hence the term granite. Granite and other granite-like stones are formed of hard minerals such as quarts, feldspar and mica, which are fused together into a very hard stone ideal for kitchen counters because its polish is resistant to household acids such as citrus and vinegar and is hard enough to resist scratching from knives and pots and pans.
True geological marble is limestone that has been subjected to great pressure and heat, which has changed its structure to a crystalline, sugary texture. It is generally white or whitish, sometimes translucent, with some veining or color provided by other minerals present at its formation.
Commercially, the term “marble” applies to any compact limestone that will take a polish, which includes most of the colored marbles, except some of the greens.
Limestone is sedimentary rock consisting mostly of organic material such as skeletons and shells of marine creatures and sediments. It is formed by material which settles to the bottom of bodies of water, and over millions of years, solidifies into solid rock.
Travertine is limestone, in a sense. It is formed by geysers, like Old Faithful, when the extremely hot underground water dissolves the underlying limestone and carries it upwards with the geyser water. When the water falls to the ground and evaporates, it leaves behind the dissolved limestone which re-hardens into stone. The new stone is full of gas bubbles, which give travertine its characteristic appearance. When it is manufactured as tiles or slabs, travertine is generally filled with cement and polished or honed.
Onyx, like travertine, is the result of water dissolving existing limestone and re-depositing it as a new kind of stone, sometimes called sinter. In limestone caves, onyx is formed by drip water, as stalagmites and stalactites. It is a very soft stone, and somewhat brittle, and needs to be installed where it will not be subject to hard wear. This beautiful stone is characterized by its translucence, and can actually be backlit for striking, dramatic effects.