Krasnyy Agat aka Red Agate - USSR (1973)
23 minutes (probably TV)
Two children explore rough seaside country, trying to reach an arched formation offshore. The boy loses his nerve but the girl swims out to it and collects a piece of red agate. She's exhausted and barely makes it back to shore - but she still has the stone.
The girl is a real cutie - I love green eyes

. I wish I knew the Russian language - it would have been helpful but I understood enough from context.