NATURAL STONE GUIDE—Igneous
Formed by cooling, or solid-state transformation of molten, or semi-molten material in the Earth’s upper mantle or crust. Most igneous stone used in dimensional stone applications is classified as continental stone, meaning that it is from the highest layer of the mantle, generally formed between 3 – 25 miles deep. Because it is formed under tremendous pressure and relatively high heat, igneous rocks tend to be very strong and dense.
Granite is the best-known example of igneous rock. Granite also has very high concentrations of quartz, making it extremely hard and difficult to scratch. Because of this formation process, granite is heat-resistant (under 1,500° Fahrenheit), acid and base resistant, extremely strong, and very scratch resistant.
This of course is a very basic overview of how the various types of stone are formed. There are also exceptions to the rule. Specifically, stone can sometimes be classified as different types.