This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The essential components of the true granites are quartz and potash feldspar. Granites are rendered complex, although the essential minerals are but two in number, by the presence of numerous accessories which essentially modify the appearance of the rocks; and these properties render them important as building stone. The prevailing color is some shade of gray, though greenish, yellowish, pink, and deep red are not uncommon. These various hues are due to the color of the prevailing feldspar and the amount and kind of the accessory minerals. The hardness of granite is due largely to the condition of the feldspathic constituent, which is valuable. Granites of the same constituents differ in hardness.
Granites do not effervesce with acids, but emit sparks when struck with steel. They possess the properties of strength, hardness, and durability, although they vary in these properties as well as in their structure. They furnish an extensive variety of the best stone for the various purposes of the engineer and architect. The crushing strength of granite is variable, but usually is between 15,000 and 20,000 pounds per square inch.