'I can still tell a real diamond from a fake'

No wonder Simon Ambrose was hired by Alan Sugar in The Apprentice, now into its fourth series on BBC1. He has been in business – or, at least, on business premises – since he was three, the age when his divorced father began taking him round his chain of discount jewellery shops.

"My old man was a diamond dealer and member of the London Diamond Bourse. He was one of the first people in jewellery discount retailing, before Ratners; he copied the idea from a man in Newcastle.

"At his peak, he had a chain of shops – three of them in Oxford Street – and a factory in Birmingham. He had queues round the block when one of them opened. So a lot of my childhood was spent being dragged from shop to shop. When I was only knee-high, I remember going through darkened rooms full of boxes of 99p earrings."

At the age of seven, he began helping out in the shops. This included taking cash from customers and putting it in the till. Most of the cash, that is: "Sometimes, I would take 20p for my pocket money." (Does Sugar know about this?) The shops also stocked individual rings worth hundreds of pounds – "the point was to give a choice" – and young Simon would slip them on, and off, his finger.

He can still remember how to distinguish a real diamond from a "CZ" (cubic zirconia, ie artificial) one: "Genuine diamonds cut glass. They have imperfections and a warmer sparkle; CZ ones are brighter and the rainbow sparkle reflected off them too perfect."

But the jewel in the crown of the experience was not the jewellery: "The work gave me a commercial rooting from the outset. I learnt to spot a good deal from a bad deal. It's the same in all businesses: telling a good egg from a bad egg. People don't walk into shops unless they're interested, and it's up to the salesman to recommend what he or she thinks they should buy.

"My father opened a lot of shops at a pivotal period. He got out when Argos moved in, and now a lot of value-for-money jewellery is made in the Far East; my father supplies Argos."

By the time Ambrose Sr left retailing, Ambrose Jr was an 11-year-old veteran. Jewellery ran in the family – his grandfather had a stall on the Portobello Road – and during school holidays, Simon helped out at two other jewellery shops, one run by an uncle in Colchester, the other by an aunt in Brighton. He retired from this glittering business at 15 to concentrate on school, and then university. Now in property, at least he can still genuinely claim to have been a diamond geezer.

Simon Ambrose is running in the London Marathon on behalf of Breast Cancer Campaign ( www.justgiving.com/apprentice)