der Kiesel
pebble
A small stone rounded and smoothed by water, like those found on beaches, in rivers, or on paths.
Barfuß über die spitzen Kiesel zu laufen war unangenehm.
Walking barefoot over the sharp pebbles was unpleasant.

pebble
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pebble
A small stone rounded and smoothed by water, like those found on beaches, in rivers, or on paths.
Barfuß über die spitzen Kiesel zu laufen war unangenehm.
Walking barefoot over the sharp pebbles was unpleasant.

'Der Kiesel' (pebble) is related to 'der Kies' (gravel). While 'Kies' describes the uncountable material (gravel, loose stones), 'der Kiesel' is the individual, countable stone from it.
Sie hob einen besonders schönen Kiesel vom Boden auf.
She picked up a particularly beautiful pebble from the ground.
'Kiesel' (pebble) is often described with adjectives such as 'rund' (round), 'glatt' (smooth), 'spitz' (sharp), or 'bunt' (colorful). Pebbles are typically found in certain places, e.g. 'am Strand' (on the beach), 'im Flussbett' (in the riverbed), or 'auf einem Weg' (on a path).
Die Kinder sammelten am Flussufer bunte Kiesel.
The children collected colorful pebbles on the riverbank.
The word is very often used in the plural ('die Kiesel' — the pebbles) to describe a surface covered with these small stones, such as a pebble beach or a pebble path.
Der Weg zum Haus war mit feinen Kieseln bedeckt.
The path to the house was covered with fine pebbles.
stone
'der Stein' and 'der Kiesel' both refer to a piece of rock. 'Stein' is the more general word and can be small, large, angular, or round. 'Kiesel' is more specific: it is small and usually rounded smooth by water, for example by a river, on a beach, or on a path. If the shape and size are not important, 'Stein' is almost always suitable. If you specifically mean a small round stone, 'Kiesel' sounds more natural.

Am Fluss sammelt Lena glatte Kiesel und legt sie in ihre Tasche.
Lena collects smooth pebbles by the river and puts them in her bag.
Auf dem Wanderweg lag ein großer Stein, über den Max fast gestolpert wäre.
There was a large stone on the hiking trail that Max almost tripped over.