The opposite of "most igneous" would be "least igneous," which refers to rocks that have not undergone extensive heating and solidification. These rocks, also known as sedimentary rocks, are formed from the accumulation of sand, mud, and other debris that is compressed and cemented over time. They are usually found in areas that have low levels of volcanic activity, such as riverbeds, seabeds, and desert plains. Another antonym for "most igneous" could be "metamorphic," which refers to rocks that have undergone intense pressure and heat but not to the extent of melting. These rocks are formed from changing existing rocks due to the movement of tectonic plates or volcanic activity.