Common Granites   

A plutonic rock may be classified mineralogically based on the actural proportion of various minerals of which it is composed.

True granites according to modern petrology are dominated mineralogically by subequal amounts of quartz, alkali feldspar, and plagioclase feldspar, which commonly collectively comprise around 90% of the rock and give them a light colour.  Relatively small amounts of dark-coloured biotite and/or amphibole provide additional descriptions (e.g. biotite granite).  Black granites usually contain little or no quartz, rare alkali feldspar, and typically much more calcic plagioclase feldspar.  Biotite and amphibole are usually present in only small to trace quantities; instead, calcic pyroxene and, less-commonly, Ca-poor pyroxene dominate the ferromagnesian mineral assemblage. Apart from these fundamental mineralogical differences ther are many other gross differences between "true" granites and "black" granites.  However, in this imperfect stone trade the two carieties are accepted worldwide under the unbrella of granite because of their igneous mode of formation, degree of crystallinity, and interlocking textures.

Classifications:

Anorthosite is a phaneritic, instrusive igneous rock (i.e. labrador Antique, Volga Blue) characterized by a predominance of plagioclase feldspar (90-100%), and a minimal mafic component (0-10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Basalt is a fine grained vesicular rock of volcanic origin, dark grey, dark green, brown, reddish or black in colour. Basalt may be compact or vesicular (i.e. porous because of gas bubbbles contained in the lava while it is solidifying).

Charnockite is a series of foliated metamorphosed igneous rocks (i.e. Verde Butterfly, Uba Tuba).  The charnockite series includes rocks of many differnet types, some being acid and rich in quartz and microcline, other basic and full of pyroxene and olvine, while there are also intermediate varieties corresponding mineralogically to norites, quartz norites and diorites.  A special feature recurring in many members of the group, is the presence of strongly pleochroic, reddish or green hypersthene. Many of the minerals of these rocks are schillerized, as they contain minute platty or rod-shaped enclosures, disposed parrallel to certain crystallographic planes or axes.  The reflection of light from the surfaces of these enclosures give the minerals often a peculiar appearance, e.g. the quartz is blue and opalescent, the feldspar has a milky shimmer like moonshine.

Diorite is a medium to coarse grained phaneritic (visible crystallized) plutonic rock (i.e. ) Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), hornblende, and/or pyroxene.  It can also be black or bluish-grey, and frequently has a geenish cast.  It is often descibed as "salt and pepper" when composed largely of light coloured minerals randomly interspersed with dark minerals.

Dolerite is a fine texture, dark-grey to black igneous rock. (i.e. Absolute Black, Swedish Black), composed mostly of plagioclase feldspar (labradorite) and pyroxene. While hornblende, ilmenite, aparite and biotite are their most common accessory ingredients.

Gabbro is a dark, coarse grained, intrusive igneous rock (i.e. Black Galaxy) chemically equivalent to basalt.  It is a plutonic rock, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.  Gabbro is a dense, greenish or dark coloured and contains varied percentages of pyroxene, plagioclase feldspar, amphibole and biotite.

Granodiorite is a phaneritic (visibly crystalline) intrusive igneous rock (i.e. Blanco Perla, Blanco Crystal) similar to granite. Medium to coarse grained rock that is one of the most abundant intrusive rocks.  It contains quartz and is distinguished from granite by having more sodic plagioclase feldspar than orthoclase feldspar.  It usually contains abundant biotite, mica and hornblende, giving it a darker appearance than true granites.

Granulite is a coarse grained, high grade metamorphic rock (i.e. Kashmere White, Kashmere Gold) composed primarily of pyroxene, plagioclase feldspar and accessory minerals, garnet, iron oxide and amphibole. 

Gneiss is a coarsed grained, imperfectly foliated, or layered, metamorphic rock (i.e. Giallo Veneziano, Juparana Colombo) formed by high grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.  Gneiss is characterized by altenating light and dark bands differing in mineral composition and hving coarser grains than those of schist,  The light bands of gneiss are generally composed of quartz and feldspar.  Hornblende, biotite mica, garnet, or graphite commonly from the dark bands.  Gneissses result from the metamorphism of many igneous or sedimentary rocks.

Larvikite (Alkali Syenite) is a coarse grained intrusive igneous rock (i.e. Blue Pearl) of the same general composition as granite but with the quartz either absent or present in relatively small amounts.  The feldspar component of syenite is predominantly alkaline in character (usually orthoclase), Plagioclase feldspars may be present in small quantities, less than 10%.  When present, ferromagnesian minerals are usually honeblende amphibole, rarely pyroxene or biotite.

Monzonite is an intermediate igneous intrusive rock (i.e. Autumn Brown, Sira Grey) composed of approximately equal amounts of sodic to intemediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals.  Quartz a minor consitituent or is absent; with a greater than 10% quartz the rock is termed a quartz monzonite.

Pegmatite is a very coarsed grained igneous rock (i.e. Azul Aran) that has a grain siz of 20mm or more; such rocks are referred to as pegmatitic.  Most pegmatites consist of granite that contains quartz, feldspar and mica.  Pegmatites are important because they often contain other are earth minerals and gemstonesm such as aquamarine, tourmaline, topaz, fluorite and apatite, often along with tin and tungsten minerals, among others.  Crystals over 10 meters across have been found.

Granite types: