Granite is a tough, durable rock composed primarily of three different minerals. These minerals are easy to see due to their different colors. The white mineral grains found in granite are feldspar. It is the most abundant mineral found in granite. The light gray, glass-like grains are quartz, and the black, flake-like grains are biotite or black mica.
Granite and marble. How are they different?
Marble is simply limestone that has been compressed and/or heated deep within the earth's crust. Unlike granite, marble was never molten rock, but it may have been heated and squeezed enough for the limestone grains to bend and flow. Marble is also made up of only one mineral -- calcite -- a relatively soft mineral with a hardness of "3". Calcite is a common mineral -- all limestones and nearly all seashells are made of it, but it does not occur in granite. Because marble is made entirely of calcite, it is much softer than granite.
How granite was formed.
Granite began 325 million years ago as a large, hot (1300 degrees to 1400 degrees Farenheit) mass of magma or molten rock. This magma originated when some of the earth's crust melted -- probably at a depth of about 11-12 miles. The magma rose upward and came to rest about nine miles beneath the earth. There it cooled very slowly -- taking more than a million years -- and solidified into granite. That was more than 300 million years ago.
Since then, the granite has been pushed upward, and the land above it has been removed by erosion. The result is a granite deposit that lies just beneath the earth's surface where it can be easily, safely and economically quarried.
