Beautiful natural stone was formerly used as the main component for building. These days, it is more commonly used as decorative accents on or within public buildings and homes, adding durable flair and natural beauty to our structures and spaces. Commonly used natural stone included marble, quartz, limestone and travertine. But for many, sturdy, colorful granite is the king of natural building materials. The best way to understand terminology associated with Granite is to describe where it forms, it's composition, and how it forms.
Where Granite is Formed
Granite is the major component of continental crust and is often exposed in continental shield areas and in the cores of mountain belts. Granite is a plutonic igneous rocks, meaning that it forms deep in the subsurface as magma cools and hardens.
The Composition of Granite
Granite comes in a variety of colors (pink and white are most common) and in a glittering array of textures, all of which are dependent on the original composition of the magma and the rate at which it cools. Common minerals that make up granite are quartz, feldspar, mica and amphibole. The feldsdpar is usually pink or white and blocky shaped. Quartz is often gray and very shiny. Amphiboles and mica, usually 10% of the rock or less, occur as black flecks or laths in an otherwise light-colored rock matrix.
As the magma cools, some minerals crystallize first, other next and still others last, in a process geologists call "fractional crystallization". As these minerals form from cooling magma a wide variety of rock textures can be produced. Common granite has an even-textured appearance. In some specimens a few of the minerals grow to particularly large sizes, in which case the rock is termed "Granite Pegmatite", prized for ornamental siding and kitchen counter tops. Last to form in a cooling granitic magma is quartz, which can produce beautiful cross-cutting veins against the more even background texture of the granite. Other, less common but highly sought-after granite textures include "Rapakivi Granite", where large pink feldspars are surrounded by an over-layer of white feldspar and "Orbicular Granite", a rare variety that displays spheroidal structures that form during magma cooling.