The stars benefit by being able to accessorise couture gowns with the world's finest jewellery at no cost to themselves. For their part, the jewellery companies stand to reap priceless worldwide publicity.

Celebrities and jewellery firms have had an enduring symbiotic relationship that has grown with time.

'When we work with a celebrity, it is a real partnership. Their personal style inspires us and plays a part in what we see them wearing,' says Sandrine de Laage, vice-president of design at Harry Winston. 'A celebrity who wears Winston becomes part of the history of the brand and a part of the Winston family.'

Harry Winston jewels have graced the likes of Marilyn Monroe, Elizabeth Taylor and Katharine Hepburn, along with contemporary stars such as Halle Berry, Scarlett Johansson and Angelina Jolie.

At the Hong Kong Film Awards last year, Lin Chi-ling wore Harry Winston Shinde briolette earrings and a lattice bracelet, the latter of which is valued at HK$1.52 million.

Chopard has also been a major player in outfitting celebrities with jewellery. It has been an official partner at the Cannes International Film Festival since 1998.

'At Cannes, we have a floor at a hotel with suites reserved for jewellery. The stars come and choose the pieces they want to wear,' says Roland Buser, managing director of Chopard for Greater China.

Celebrities who have worn Chopard jewellery include Helen Mirren, Penelope Cruz, Mariah Carey, Elton John, Michelle Yeoh and CoCo Lee.

Top-of-the-line, one-of-a kind jewels are the rule for red carpet occasions. 'These are unique pieces that can cost from US$3 million to US$10 million,' Buser says.

Normally, a star's representatives contact Chopard, not the other way round. It helps if the celebrity makes a commitment to Chopard. 'Loyalty is a value for us. It doesn't really work in our favour if the celebrity switches brands year after year,' Buser says. 'There's no continuity then.'

The company is also sensitive about a star's image. It deals with people who are passionate, positive and down to earth, avoiding those with a negative image.

'If we are interested, they tell us what type of dress they will be wearing, the colour and the kind of jewellery they are looking for. They pretty much know what they want. We draft a proposal and suggest several pieces and they make their choice,' he adds.

Buser says the jewellery requests run the gamut of styles, from big bangles to minimalist pieces and from earrings only, with nothing around the neck, to big pendants. 'Some people want a lower profile and some want an extremely high profile. It really depends on the mood and the dresses.'

All the jewellery is sourced from Chopard's existing collections. Modifications for one-time outings on the red carpet are not allowed because of the unique nature of each piece.

'Gemstones account for 80 per cent of the value of an item and it sometimes takes years to find them,' Buser says, although the modification restriction is waived for actual buyers.

Having a parade of stars photographed wearing the company's high jewellery is worth its weight in gold.

'It's definitely about branding. There is a huge benefit from exposing ourselves with these superstars.

'The return on investment is a lot higher than placing an ad in a newspaper,' he says.

The ultimate goal of the publicity, however, is attracting buyers. Celebrities are a means to that end.

And, no, they don't get to keep any of the fabulous jewels. 'We do appreciate it if they return them,' Buser smiles.