ntroduction: What Is an Igneous Rock? gneous rocks vary greatly. Some contain large mineral grains. Others contain grains so small they can barely be seen under a high power microscope. gneous rocks also vary greatly in color. All igneous rocks are formed through the cooling and solidification of magmaIntroduction: What Is an Igneous Rock? ◼ Igneous rocks vary greatly. ◼ Some contain large mineral grains. ◼ Others contain grains so small they can barely be seen under a high power microscope. ◼ Igneous rocks also vary greatly in color. ◼ All igneous rocks are formed through the cooling and solidification of magma