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?