der Felsen
rock
A large, massive rock formation that protrudes from its surroundings, for example on a coast or in the mountains.
Die Wellen brachen an den schroffen Felsen der Küste.
The waves broke against the rugged rocks of the coast.

rock
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rock
A large, massive rock formation that protrudes from its surroundings, for example on a coast or in the mountains.
Die Wellen brachen an den schroffen Felsen der Küste.
The waves broke against the rugged rocks of the coast.

This word belongs to the N-declension. This means that the nominative singular is 'der Fels' (the rock), but in all other cases (genitive, dative, accusative singular and throughout the plural), the form 'Felsen' is used.
Ein großer Fels blockierte den Weg.
A large rock blocked the path.
In its literal meaning, 'Felsen' (rock/rocks) is often combined with adjectives such as 'schroff' (rugged), 'steil' (steep), or 'massiv' (massive). Typical actions are 'auf einen Felsen klettern' (to climb onto a rock) or 'an einem Felsen stehen' (to stand by/on a rock).
Die Bergsteiger kletterten den steilen Felsen hinauf.
The mountain climbers climbed up the steep rock.
The idiom 'ein Fels in der Brandung sein' (to be a rock in the storm/surf) is a fixed expression. It is always used together with the verb 'sein' (to be) to describe a person who is very reliable and calm.
Während des ganzen Projekts war er unser Fels in der Brandung.
Throughout the whole project, he was our rock in the storm.
stone
'der Stein' and 'der Felsen' both belong to the area of rock. 'Stein' means a single piece of rock and can be small or medium-sized. You can often pick up or throw a 'Stein'. 'Felsen', on the other hand, means a large, massive rock formation in nature, for example in the mountains or on the coast. The words can be confused because both are made of similar material. As a simple guide: a 'Stein' lies on the path, while a 'Felsen' stands or juts out in the landscape.

An der Küste sitzen die Möwen auf einem hohen Felsen.
On the coast, the seagulls are sitting on a high rock.
Auf dem Schulweg liegt ein kleiner Stein in meinem Schuh.
On the way to school, there is a small stone in my shoe.