This fact indicates that the Earth's crust is composed of a relatively small number of common elements, and that these elements tend to combine in specific ways to form the minerals that make up rocks.

Here's a breakdown:

* Limited building blocks: While there are over 5,000 known minerals, only a few dozen are truly abundant in rocks. This is because the Earth's crust is primarily composed of a few key elements like oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.

* Stable combinations: These elements readily combine to form a relatively small number of stable mineral structures, like quartz (SiO2), feldspar (various compositions), and mica (various compositions).

* Rock-forming minerals: These common minerals make up the vast majority of rocks, both igneous, sedimentary, and metamorphic.

Therefore, the fact that only a few minerals are common in rocks reflects the limited elemental makeup of the Earth's crust and the tendency of these elements to form stable mineral structures.