Fifteen years ago the Lam family business picked up a consignment of aged tea from a defunct Hong Kong restaurant. Its value has since risen by a factor of 10,000, as the Lams have found themselves part of a boom that is both investment fad and cultural obsession.

"It's like magic," managing director Sam Lam told AFP as he prepared tea according to the Chinese ritual, pouring boiling water through rough leaves and then into tiny cups to drink, and spoke of the profits to be made.

The tea is pu erh, a dark tea that is fermented after drying and whose taste mellows with age. Its history is thought to date back between one and two thousand years, with legends of growers in mountainous Yunnan province ferociously guarding their cultivation secrets.

Over the past 20 years prices for aged pu erh have rocketed, while China has encouraged renewed development of a luxury tea culture which parallels that of wine -- partly as a source of national pride in a home-grown high-end product.

With over 70,000 tea businesses in mainland China, skilled buyers must taste tea in order to assess its quality, which only increases pu erh's mystique and sociability.

"You can tell from the aftertaste, the smoothness," says Lam, pouring out cups with practised hands. The tea is sold in pressed round "cakes", wrapped in paper printed with bold designs that reflect the vintage of each one.

Lam's father set up the business, Lam Kie Yuen, after moving to Hong Kong from the war-torn mainland in 1949.

But the pair say it is only since the mid-nineties that the market for luxury pu erh -- also, in its less refined forms, a staple of cheap restaurants -- has exploded, with middle-class investors joining the wealthy to buy it up.

The Lams are now selling tea from the 1930 to 1950 era for up to HK$200,000 (over $25,000) per 345-gram (12.2 ounce) cake, having bought much of it in cheap truckloads from dim sum restaurants that closed down.

"Growth slowed during the economic downturn, but it's still ongoing," said Sam Lam. "As the price is rising, people are buying it less to drink, and more to collect and invest."

But luxury pu erh is not just bought to lay aside; it is identified with proud, ancient aspects of Chinese culture, in contrast with the cheap "made in China" goods that have spurred the country's economic rise.

In Hong Kong's hectic Mong Kok district, fashionably dressed young men gather at a calm tea house for lessons from qualified tea master Eliza Liu.

"It's like a drug -- I'm addicted now," said student Ngan Kan Shing, 21. "By discovering tea I feel that I've learned about China."

He has been coming to classes for six years, but says: "I still only know the basics."

The group examines the colour of each cup of tea before sniffing and then slurping it in respectful silence, as Liu talks them through the value of the aged tea.

Grown before artificial pesticides and dried naturally rather than at a high temperature, it has a paler colour and a smoother taste.

"Good tea is produced at higher altitude, and also depends on climate," says Liu. "In Yunnan, they say a tea tree can experience all four seasons in one day."

The tea is served from small fine china tea sets, used with a tray that drains off excess water. The first cup of each brew is not drunk, as it is used to clean dust or residue from the leaves. After that, a good tea should taste different with every cup, say experts.

According to China's state-run Global Times, one batch of top pu erh sold at auction for $250 per gram in 2002, while rare Da Hong Pao oolong can also rival such prices.

But Liu and tea professor Yip Man, who taught her the art, are sceptical of the eye-popping prices paid for some teas, preferring to emphasise tea's longtime role in Chinese medicine and thought.

"Tea has a philosophy behind it, and it's about health. Tea has been very commercialised, but a cheaper tea may also be as good (as an expensive one)," said Yip.

"The philosophy is about harmony, bringing people together, peace within the self."

The price of pu erh is acknowledged to be boosted by a tight supply, and sceptics argue that investors buying aged pu erh may be made to look foolish as China's newly affluent drinkers move on to fresh fashions like Phoenix Oolong.

However Lam says that although the astonishing growth of the last two decades may not be sustained, pu erh is still a good investment.

He said a buyer of a good, inexpensive pu erh -- at, say, HK$100 ($13) a cake -- now could expect to make a 10 percent return in a year. "But you have to choose the right tea," he said.

Luxury tea houses springing up in London and Sydney indicate China might succeed in exporting its high-end tea culture. And Liu and her students feel meanwhile that they are tapping into much more than a fad.

Student Ngan is evangelical about pu erh. "Before learning about tea, a lot of my friends believed the stereotype that tea is for old people. But now I think they're changing their minds," he said.