石 and 岩: stones and larger rocks
Describe size, location, and material only to the extent stated.
Picking up or placing stones
石 (いし) means stone. 「小さな石を拾いました」 means picking up a small stone.
「石を並べて道を作りました」 describes making a path by arranging stones. 石 gives the material used for the path.
A larger rock mass
岩 (いわ) refers to a large rock or rock mass. 「大きな岩の陰で休みました」 means resting in the shade of a large rock.
「岩の上に鳥が止まっています」 describes a bird perched on a rock. The action is not limited to human sitting on a chair.
Avoiding an exact size rule
Do not fix the everyday distinction between 石 and 岩 at one exact number of centimeters. Size can be added separately, as in 「大きな石」.
「石でできた橋」 describes a bridge made of stone. で marks material, unlike a sentence locating stones on a bridge.
What does 「石でできた橋」 describe?
Show the answer and why
Stone as the bridge's material, not the location of stones.