What key observation led to the discovery of magnetism by the Greeks?

The discovery of magnetism by the Greeks was primarily influenced by the observation of magnetite, a naturally occurring magnetic mineral. Specifically, they noted that magnetite had the ability to align itself in a consistent direction when undisturbed. This property became evident when pieces of magnetite, often referred to as lodestones, would naturally orient themselves along the Earth's magnetic field. This foundational observation helped establish the principles of magnetism and led to further inquiries about magnetic forces and materials.

Understanding this historical context is essential, as it highlights how empirical observations can lead to significant scientific discoveries. Other options, while interesting, do not reflect the specific observations made by the Greeks regarding magnetism's fundamental nature.