Granite

Granite is the most familiar coarse-grained (phaneritic) intrusive igneous rock, instantly recognizable by its salt-and-pepper speckled appearance. It is composed chiefly of quartz, alkali and plagioclase feldspar, and mica (biotite or muscovite), often with minor amphibole.

Because it cools slowly deep underground, its mineral grains grow large enough to see and identify with the naked eye. Granite forms the core of the continental crust and is exposed in mountain ranges and ancient shields worldwide.

Durable, hard, and attractive when polished, granite is one of the most widely used building and dimension stones on Earth, though many commercial 'granites' are technically other rock types.

Granite forms when large bodies of silica-rich (felsic) magma cool and crystallize slowly at depth within the crust, typically several kilometers down. The slow cooling allows interlocking crystals of quartz and feldspar to grow large.

These magma bodies, called plutons or batholiths, are usually generated in continental collision and subduction settings where the crust is thickened and partly melted. Over millions of years, erosion strips away overlying rock to expose the granite at the surface.

Classic exposures include the Sierra Nevada batholith and Yosemite (USA), Dartmouth and Cornwall (England), the Canadian Shield, Scandinavia, and vast areas of Brazil, India, and South Africa.

Look for a hard, coarse-grained rock with visible interlocking crystals and a speckled 'salt and pepper' look: glassy grey quartz, blocky white-to-pink feldspar, and shiny black or silvery mica flakes.

The interlocking, randomly oriented crystals (no layering or banding) distinguish granite from metamorphic gneiss, which shows banding, and from layered sedimentary rocks. Overall hardness is high; quartz and feldspar both scratch glass.

Look-alikes include gneiss (banded, foliated), diorite (less or no quartz, darker), and gabbro (dark, no quartz, no visible pink feldspar). True granite always contains abundant visible quartz plus pale feldspar, giving it a generally light overall color.

Granite is a premier construction and dimension stone, used for countertops, flooring, facades, monuments, curbstones, and tombstones thanks to its hardness, durability, and polish-taking ability. Crushed granite serves as road base, railroad ballast, and concrete aggregate.

Historically it has been used for everything from Egyptian obelisks and statues to bridges and public buildings, valued for resistance to weathering and abrasion.

Geologically granite is enormously significant as the dominant rock of the continental crust and a key to understanding mountain-building and magma processes. Note that the stone trade labels many unrelated rocks 'granite', so commercial granite is not always true granite.