The main difference between granite and quartz is that granite is a rock and quartz is a mineral. In geology, rocks are materials that are composed of many different minerals. Minerals, however, are composed of a definite chemical composition.
Granite is a type of igneous rock, which is formed when magma comes up from deep within the earth and hardens near the surface but not through the surface. The magma becomes granite when it cools slowly enough to allow quartz crystals to grow big enough that they can be seen with the naked eye. Feldspar, mica, amphiboles and other minerals are also found in granite. Depending of the ratio between mineral in a sample, the granite can be tinged red, pick, gray or white. Sometimes, dark veins of minerals coursing throughout the sample can also be found.
Quartz is considered the most abundant mineral on earth and is found in igneous, metamorphic and sedimentary rock. Quartz is a natural form of silicon dioxide and, depending on other molecules found within the magma from which it forms, can be found in a range of different colors. Silicon dioxide particles also form in a set pattern, so many quartz crystals form into six-sided, pyramidal shapes. These shapes can be short and stubby, such as the quartz found in a geode, or long and prismatic. Quartz crystals are named for their color. For example, amethyst quartz is purple, citrine is red-yellow and chrysoprase is green.