Most women have certain products they swear by - the moisturiser they've used for 20 years; the mascara they feel naked without, or a shade of lipstick which is their signature - but certain products go beyond just personal preference and have entered a new realm. Mentioned in elite lists of beauty essentials, name-checked by actresses and super-models, sold almost purely by word of mouth, these are the silent best-sellers; the products that have become cult items. There are no hard-and-fast rules as to why certain products become beauty icons and others don't, but there are certain necessities.

First of all those name checks; when the name of a product is mentioned by somebody famous and glamorous, it is more valuable than the most expensive ad campaign. When model Linda Evangelista mentioned that she wouldn't leave the house without a Mac Spice lipliner at the height of her fame in the 1980s, it sparked off a craze for that particular shade that lasted until recent years. The sales figures of the Yves Saint Laurent Touche Eclat highlighter, pictured above, were achieved with the help of celebrity fans including Victoria Beckham, Kate Moss and Kate Winslet, while another cult product, Creme de la Mer moisturiser, is beloved by Jennifer Lopez, Uma Thurman and Heather Locklear.

This is the kind of publicity that money can't buy - it's one thing to use Liz Hurley for your press shots; it's quite another to have a superstar confide her own beauty secrets. Who could resist the suggestion that you too could achieve a similar look if you just start buying the cream?

Two other factors also contribute to a product's beauty status; availability and price. Creme de la Mer costs a phenomenal $85 per ounce or $1,000 for 16oz, and yet the few stockists worldwide have waiting lists of women desperate to get their hands on a tiny pot. Expect the same when it arrives in Brown Thomas for the first time in March 2001. A facial by beautician Eve Lom is another cult treatment, and one which doesn't come cheap or easy: there's a three-month waiting list for an appointment with Lom herself, while her cleanser cream, available exclusively at Blue Eriu, comes in at a cool £55.

Of course, anything which is only available in the US or Japan is often guaranteed instant culthood, regardless of its price. For years, jet-setters would subtly show off their air miles by using Maybelline's Great Lash mascara - a cheap product, used by many models, which was only available in the US. As soon as it was readily available here, it lost a little of its cachet. Cheap and cheerful Carmex lip salve, in its distinctive yellow tin, and vicious-smelling Noxema cleanser also qualify for culthood by virtue of their unavailability in Britain or Ireland. Here, price is irrelevant - they're cult products because they're good but rare.

Cult status can also be inspired by the myth attached to a potion's production. The makers of Creme de la Mer can demand such big bucks because the production process is elaborate in the extreme. The cream, which was created by a NASA scientist Max Huber to treat his own chemical burns, is made of a plant broth containing vitamins, minerals and a sea kelp which is harvested according to lunar cycles. If this wasn't enough mumbo-jumbo, the broth is also played sound recordings of previous fermentations and has pulsing light shot through it while it ferments.

Then there's Shu Uemura's Depsea waters which are created from water taken from thousands of metres beneath the surface of the sea.

Yet certain products which are readily available at reasonable prices, such as Touche Eclat, Clarins Beauty Flash Balm, or Elizabeth Arden's Eight Hour Cream also undoubtedly qualify for cult status. Similarly, very exclusive products can remain obscure and beloved only by a few. So what is it that finally makes a product into a cult?

"They work, that's why," says Tara McHugh, a make-up artist and firm fan of Touche Eclat, Eight Hour Cream and Nars Multiples stick. "These products become cults because they actually work, whereas other products make great claims for themselves but just aren't great." She regularly uses the highlighter to even out skin tone, the cream for everything from cracked lips to eyeshadow slick, and the multiples for bronzing eyes, cheeks and lips. "They're quick, they're versatile and you can shove them in your handbag and go."

Sometimes, a cult can be a simple matter of practicality.