Marble is a type of metamorphic rock. The calcite in limestone melts when heat and pressure is applied to limestone. Large marble crystals form as a result of this heat and pressure. Whatever chemicals were in the limestone while marble was forming dictate the ultimate color of the marble. Granite is an igneous rock that is made up mostly of quartz and feldspars. The quartz in granite is what gives it its superior strength to most other stones.
According to Countertop Specialty, marble countertops are softer than granite countertops. Both materials are very hard and durable. They are both much less susceptible to cracking or breaking compared to other types of countertops such as wood and ceramic tile.
Marble is a relatively porous rock, which means that it is prone to staining. Excessive heat and overexposure to acids can cause a marble kitchen countertop to rapidly deteriorate. Granite, on the other hand, is quite resistant to heat and acid-wear.
Granite and marble remain some of the most expensive stone materials used in the construction of kitchen countertops. Their cost per square foot usually only differs slightly. In 2010, granite countertops range in price from $50 and $90 per square foot installed, according to Keidel. Marble countertops are similarly priced at between $40 and $120 per square foot installed, according to Cost Helper in 2010. Marble prices can vary dramatically by region.
Part of the reason that marble is traditionally a little more expensive than granite is that marble is not as widely available as granite across the globe. According to Think Quest, marble is mined in the mountainous areas of Canada, Germany, Italy and Spain. Vermont, Tennessee, Maryland and many other states in America have marble mining operations; some marble mined from these locations is used for the sole purpose of manufacturing kitchen countertops. Granite is prevalent across the globe; many nations actively mine for granite.
Both granite and marble remain some of the strongest, most durable types of kitchen countertop available to consumers. The a big difference, however, is in their overall hardness. According to the Moh's Scale of Hardness, the calcite in marble gives marble an overall hardness rating of "3" and the orthoclase or quartz in granite gives it a hardness rating of about "6." Diamonds are the hardest materials on Earth; they feature a hardness rating of "10."