Granite is a plutonic rock that is composed of between 10 to 50% quartz

Granite is an igneous rock made up of primarily quartz, feldspar, micas, amphiboles, and a mixture of additional trace minerals. 

These minerals and their variation in abundance and alteration give granite the numerous colors and textures we see in granite countertops.

Formally, granite is a plutonic rock that is composed of between 10 to 50% quartz (typically semi-transparent white) and 65 to 90% total feldspar (typically a pinkish or white hue).

Granite is an intrusive igneous rock, which means it was formed in place during the cooling of molten rock. Generally, the slower the molten rock cooled, the larger it's mineral crystals with K-Feldspar megacrysts forming in special circumstances greater than 5cm. 

During formation of granite it is buried below kilometers of rock and sediment necessary to produce enough heat to melt rock. Of course, the granite we see today is near surface, and thus at some point was uplifted, causing overlying sediment to be shed via erosion. 

This transition from high pressure and temperature to atmospheric temperature and pressure can cause the granite to slightly expand and crack. 

This, in addition to seasonal variations in temperature can leave you with a weakened and less desirable granite to use for countertops.

What Is Quartz And How Are Quartz Countertops Made?

You may be a bit confused in that there are granite countertops and quartz countertops. As discussed above, granite is a type of rock that is partially composed of quartz, a mineral. 

Therefore, what exactly is a quartz countertop? You may initially think that a quartz countertop would be composed of a quartz arenite, a rock made up of at least 90% quartz.

This may remind you of a white sandy beach with a few brown and pink specs. The natural sorting of quartz on beaches is driven by mineral strength (I'll get to that below) and their differential weathering rates. 

As weaker minerals weather and erode relatively quicker than quartz, you are left with quartz as a predominant mineral in well weathered sediment systems (i.e. beaches) far from sediment source (i.e. mountains).

However, you will find that quartz countertops are actually man made and although they consist of 90% quartz, the rest of the countertop is composed of resins, polymers, and various types of pigments. 

This creates a very durable artificial rock without pores or cracks. The pigment is used to provide coloration that is naturally found in quartz arenite.

Comparison of Granite vs. Quartz Countertops

Here, I'll directly compare both granite countertops and quartz countertops with physical, chemical, and geologic parameters. Where noted, I use quartz arenite instead of the composite quartz countertop due to lack of data on the quartz composites.

	Quartz (Quartz Arenite)	Granite
Mohs Hardness	7 (pure quartz)	6-7 (depending on mineralogy)
Melting Point	1670 °C	1215–1260 °C (dry)
Natural Radiation	None	Trace amounts
Source	Man-made	Natural
Natural Fractures	None	Small but variable amounts
Porous	No	Very slightly

Should You Be Concerned About Radon From Granite?

Another important aspect when considering the two options is radon and unfortunately there is a variety of misleading or scare-mongering information online. Radon is a radioactive gas that is naturally found in granite due to the decay of radium to radon.

Granites can contain parts per billion of radium, providing for an extremely small amount of radioactive material to decay and produce radon. 

Based on significant research from granite's around the world, it is determined that there is a minimal and acceptable amount of radon in average granite building materials.

Although quartz countertops will have less to zero radon, it does not preclude granite as a "less safe" choice in kitchen countertops. 

Given the radiation naturally encountered through flying, walking around a city, and living your everyday life, it is not significant enough to discourage granite in the household.

https://www.forbes.com/sites/trevornace/2015/10/26/quartz-vs-granite-countertops-geologists-perspective/#6701ca00196c