Granite is one of the most commonly known types of rocks, used in everything from buildings to sculptures. It has been used for thousands of years and is regarded as a symbol of status, strength, and durability.
What is Granite?
Granite is an intrusive igneous rock with large grains (minerals) easily seen by the naked eye. Granite colors are most commonly pink, white, variations of grey and black. However, it's important to note that some stones marketed as black 'granite' are in fact likely gabbro as granite must contain at least 20% quartz within a rock to make it granite.
Now, let's break down what exactly an intrusive igneous rock is:
· An intrusive rock means that molten rock cooled within the crust and was never expelled as molten rock. The gradual cooling of molten rock is imperative to create the large crystals of a singular mineral that we see in granites. With time, there is differential lithification or solidifying of molten rock dependent on chemical makeup, this allows for different types of minerals to form at different periods of time and alter the final resulting granite. Therefore, the size of individual grains is proportional to how slowly the molten rock was cooled. Extrusive rocks cool during a volcanic eruption and allow no time for orientation of minerals, creating a homogenous looking rock with no discernible grains.
· An igneous rock is a rock that has solidified from molten rock. This is in comparison to the two other major types of rock, sedimentary and metamorphic.
What Determines Granite Colors?
Granite is a conglomerate of minerals and rocks, primarily quartz, potassium feldspar, mica, amphiboles, and trace other minerals. Granite typically contains 20-60% quartz, 10-65% feldspar, and 5-15% micas (biotite or muscovite). The minerals that make up granite give it the unique colors we see in different types of granite.
The relative proportion of different colored minerals in a granite is largely due to the original source of molten rock that cooled to form the granite. If the molten rock was abundant in potassium feldspar, the granite is more likely to take on a salmon pink color. On the other hand, if the molten rock is abundant in quartz and minerals that make up amphibole, you will likely get a black and white speckled granite commonly seen on countertops.
· Quartz - typically milky white color
· Feldspar - typically off-white color
· Potassium Feldspar - typically salmon pink color
· Biotite - typically black or dark brown color
· Muscovite - typically metallic gold or yellow color
· Amphibole - typically black or dark green color
The combination of the minerals above make up most of the colors we typically see in a granite. Now, let's break down the distance types of granite and a quick overview of what gives them their color
White granite is a granite that is composed primarily of quartz (milky white) and feldspar (opaque white) minerals. The small black specks in the granite above are likely small amphibole grains. This could be due to a lack of chemical components needed to form amphibole, or the cooling process was not amenable to formation of amphiboles.
If you see a rock that is 100% white, it is not granite but more likely a man-made rock that is created to look like granite or a quartz (quartzite) countertop.
"Black granite" is commonly seen in commercial rock, but it is not granite at all. As said above, granite must be at least 20% quartz, which means an all black rock is not a granite. Most commonly, black granite is in fact gabbro, a mafic intrusive igneous rock similar to basalt. Gabbrois primarily composed of minerals pyroxene, plagioclase, and small amounts of olivine (dark green) and amphibole.
Pink colored granite is a result of an abundance of potassium feldspar within the granite. You can see small specs of milky semi-transparent quartz, dark brown/black amphibole, and opaque white feldspar. However, in a granite like the one above the primary mineral is potassium feldspar.
The above granite appears to have equal parts quartz, feldspar, and amphibole, making a speckled black and white granite. This is one of the most common types of granite and one that is most commonly seen used for granite countertops.
Red granite is a variation of pink potassium feldspar abundant granite, where the k-feldspar takes on a redder than pinker color. Also, you can get red coloring from iron oxide in hematite grains or inclusion within feldspar, essentially the same process that makes rusted metal ruby red colored.
You may find advertisements for blue granite countertops but this is also almost certainly not granite. One potential is that the rock is actually Larvikite, an igneous variety of monzonite and sometimes referred to as "blue granite" despite it not being granite. Another common alternative is Anorthosite, a rock that contains abundant blue labradorite and is sometimes sold as blue granite.
When advertised as green granite, often times the stone is actually a green variety of marble, which gains its green coloration due to inclusions of serpentine. It could also be a green variation of soapstone, mislabeled as granite. Granites are not abundant in green colored minerals, but there are a variety of different rock types that do contain green minerals in abundance. One very unusual way to get a green coloration is the inclusion of amazonite, a green variety of feldspar.