Georgina Le Fjord was out with a friend celebrating the end of her exams when the accident happened on Llantwit Major beach, Vale of Glamorgan, in June.
The inquest in Cardiff heard she would have died instantly.
Assistant coroner Thomas Atherton recorded a conclusion of accidental death, calling it a "terrible tragedy".
Ms Le Fjord - who grew up in Salisbury, Wiltshire, but had been living in the area - was "the happiest she had ever been" after suffering from depression, the inquest was told.
That day, she was with her friend Martha Ormerod, 25, marking the end of her A-levels when debris from a cliff gave way.
In a statement read to the hearing, Ms Ormerod said they were sat about 15ft (4.5m) away from the rocks.
"As we were sat down enjoying our picnic I heard a sudden noise behind me. I turned around and saw Georgie lying on her front," she said.
"At first I thought she had managed to dive out of the way."
Eyewitnesses tried to help Ms Ormerod administer first aid.
One, Claire Louise Hobson, said there had been a rock fall there a few days before.
"I think perhaps about three or four large rocks hit her. At the time I did not think that she was that badly hurt," she added.
PC Mark John, who later broke the news to boyfriend Paul Biggs, said it was the most distressing incident he had seen in his police career.
Ms Le Fjord's mother Jackie Cleary, 53, described her daughter as inquisitive with a passion for knowledge, adding: "The last few years of her life she was truly at her happiest."
Reaching his conclusion, Mr Atherton said the death served as a reminder of how dangerous rocks near cliffs can be, especially as they can fall "without warning".

The fire on 1 January destroyed the nursery and infant classrooms at Coed Eva Primary School.
Seven people from the area have been arrested on suspicion of arson with intent to endanger life and are currently on bail.
Crimestoppers has put up the reward as it is believed that there are local people who hold vital information.
Ian Johnston, Crimestoppers' ambassador for Wales, said: "This arson attack has hit a local community extremely hard with over 200 pupils directly affected by the destruction of their school building."
The reward will only qualify if information passed to Crimestoppers leads directly to an arrest and conviction.
A 23-year-old woman killed by a falling rock on a south Wales beach was "in the wrong place at the wrong time", an inquest has heard.
[NEXT_CONCEPT]
A reward of up to £5,000 is being offered for information following an arson attack on a Torfaen school.