What property defines igneous rocks?

Igneous rocks are defined by the process of crystallization of molten material, which occurs when magma or lava cools and solidifies. This can happen either beneath the Earth’s surface, leading to the formation of intrusive igneous rocks, or on the surface following a volcanic eruption, resulting in extrusive igneous rocks. The primary characteristic that distinguishes these rocks is their origin from molten rock, and this crystallization process leads to the formation of various minerals that can be identified in the rock structure.

The other options refer to different geological processes. For example, the formation in underwater conditions pertains more closely to sedimentary rocks, which can also involve organic materials and fossils. The layering of sediments is a hallmark of sedimentary rock formation as well. Thus, the unique property of igneous rocks lies specifically in the crystallization of previously molten material.