Most granites formed from hot, molten [magma] that slowly cooled and hardened far below the earth’s surface. Because of this slow cooling, fairly large mineral grains were formed.
Polished section of pink [granite] from Gillespie County, Texas.
Granites are now seen at the surface in several areas of Texas. They were gradually uncovered as the areas became higher and the overlying rocks slowly weathered away. One of these areas is the [Llano uplift] of central Texas where the granites occur in Blanco, Burnet, Gillespie, Llano, and Mason counties. These granites formed during [Precambrian] time and are believed to be about a billion years old. (Scientists are now able to determine the age of some rocks accurately by very precisely measuring the relative amounts of isotopes produced by decay of radioactive minerals.)
Texas State Capitol building at Austin is made of Burnet County [granite] obtained from Granite Mountain near [Marble] Falls, Texas.
Granites also appear at the surface in the Trans-Pecos country of west Texas. Some of these areas include the Franklin Mountains of El Paso County, the Quitman Mountains of Hudspeth County, the Chisos Mountains of Brewster County, and the Chinati Mountains of Presidio County.
Red, pink, and gray granites from quarries in the [Llano uplift] area are widely used as building stones and monument stones. A large quarry at [Granite] Mountain just west of [Marble] Falls in Burnet County has supplied pink granite for buildings in many parts of the United States. The Texas Capitol building and several other State buildings in Austin are made of this granite.
Graphite
[Graphite] is a mineral that is made up of a single [element]—carbon. (Diamond, although it does not look at all like graphite, is a [crystalline] form of carbon.) Graphite is a steel-gray or black mineral that commonly has a metallic luster. It is not heavy and is extremely soft. Graphite will soil your fingers and leave a black mark on paper. This mineral cleaves perfectly in one direction and splits into thin flakes that feel greasy.