When we identified minerals, we had very precise tests to perform and specific properties to measure. Identifying igneous rocks is not that simple. They are classified by what minerals they contain, and the size of those mineral pieces, which are called grains. This creates a system that is not incredibly precise. For example, to be classified as granite, a rock must have at least 10% quartz. But what if it only has 9.99% quartz? For that matter, how can you measure exactly what percentage of a rock is quartz? The different classes of igneous rocks grade into each other, with granite merging smoothly into diorite without a sharp difference.
There are over 700 different kinds of igneous rocks, but geologists group them into a few general categories. Using the chart below, you can easily identify most of the igneous rocks you find.