石

1. Origins, structure, and basic meaning

• Structure: 石 (いし) is a noun for a stone or rock. It is the picked-up object in 石を拾う and the support sat upon in 石に腰掛ける.
• Basic usage: A small roadside stone is collected, whereas a garden rock serves as a place to sit and rest.

2. Common pitfalls and similar expressions

• Stone handled and rock used as support
  • を marks a movable object, while に marks the point contacted by the body. In the second context 石 is a concrete rock, not an abstract material label.