Olivia Claffey noticed the gem was missing from the setting of her £31,000 ring.
She sparked a frantic hunt involving staff at Victoria Station in London.
Stone given as a Christmas gift by then-boyfriend Mark Hare was never found.
Mr Hare is now suing the jewellers from whom he bought the ring and stone.
A businessman is suing a jewellers' after a £29,000 yellow diamond he gave his girlfriend for Christmas was lost when it fell out of her ring.
Olivia Claffey noticed the gem was missing from the setting of her £31,000 ring as she got off a train and into a taxi at London Victoria station on March 4, 2018.
She sparked a frantic hunt and staff scoured the tracks but the precious stone - which was a Christmas gift from her then-boyfriend Mark Hare - was never found.
Insurance broker Mr Hare, from Warninglid in West Sussex, is now suing jewellers Raymond Lloyd Ltd, trading as Philip Lloyd Jewellers, who are disputing the claim.
The 58-year-old says the diamond was lost because one of the 'claws' of the setting holding the diamond in place on the ring snapped off.
His barrister Andrew Spencer told Central London County Court, 'the ring setting was not of satisfactory quality, was not reasonably fit for the purpose of holding the diamond, and that the setting of the diamond into the ring setting was not undertaken with reasonable skill and care'.
Mr Hare is claiming the diamond's market value, which he says the jewellers put at £56,950 - or at least the return of the £29,000 he paid for the rare stone.
But Philip Lloyd Jewellers are fighting the claim, insisting the ring setting was of good quality and blaming Ms Claffey's 'reckless use of jewellery' for the loss.
Judge Nicholas Parfitt was told that Mr Hare had shelled out a total of £30,750 for a yellow stone diamond ring as a special gift for his then-partner.
The 58-year-old initially bought the stone for £29,000 from Philip Lloyd Jewellers in Reigate in December 2017, later returning with Ms Claffey in the New Year when she picked out a £1,750 ring setting for it.
After the jeweller arranged for the stone to be set, Ms Claffey collected her new ring in February 2018, but says she wore the sparkler only two or three times before losing it at Victoria Station the following month.
Ms Claffey explained how the diamond disappeared from its ring setting during a day trip to London to see a friend.
She recalled looking at it on her finger and 'admiring' it while on the train. But when she glanced at the ring again as she got into a cab at Victoria, she noticed it was missing.
She started an exhaustive search at Victoria, she told the court, spending four hours in a fruitless hunt which also involved train workers checking the track for the stone.
She finally called home to give Mr Hare the bad news which she said was 'the worst phone call I ever had to make'.
'I was quite nervous because I had to say, 'by the way the £29,000 diamond you gave me for Christmas which at the time was the most beautiful and significant thing I'd ever been given was lost',' she added.
The jewellers' barrister Sally Anne Blackmore, highlighted expert evidence suggesting the diamond was lost 'as a result of heavy wear and lack of care by the wearer'.
The same expert decided that 'the most likely cause of the failure of the ring setting was wearing and taking off a glove while wearing the ring', she explained.
Ms Blackmore suggested Ms Claffey had been nervous about telling Mr Hare because she had a record for 'reckless use of jewellery' and 'was frightened because she thought he would be very angry that she had damaged another piece'.
Mr Hare said he had been a long-standing client of Philip Lloyd Jewellers and acknowledged he had always received 'excellent' service in the past.
He approached the jewellers in December 2017 and asked if they could 'source a fancy yellow diamond' which he then gave Ms Claffey for Christmas.
Ms Claffey only wore it 'on special occasions only,' he told the court.
He only remembered her wearing the ring three times, he added, including on a trip to the Metropole Hotel in Brighton and on the day of its disappearance.
Asked by Ms Blackmore whether his ex had a habit of neglecting her jewellery, he replied: 'She is not hard on her jewellery, I don't consider her any better or worse than any other young lady.'
Mr Hare's barrister argued that the claw holding the ring in place failed due to a simple defect in the ring, rejecting suggestions that Ms Claffey caused the damage by neglecting her jewel.
'The only options are misuse or a defective ring. I submit that on Ms Claffey's evidence misuse is completely ruled out,' he told the judge.
Judge Parfitt has now reserved his ruling in the case to be given at a later date.