In 2002, a wealthy purchaser paid 180,000 yuan – almost $28,000 – for just 20g of China's legendary Da Hong Pao tea. Even in a culture that’s valued tea drinking as an art form for around 1,500 years (and has a system of tea classification that makes French wine look simple), the price was astonishing.

Original Da Hong Pao doesn't just cost its weight in gold – it costs more than 30 times its weight in gold: almost $1,400 for a single gram, or well over $10,000 for a pot. It’s one of the most expensive teas in the world.

“It looks fit for a beggar, but it's priced for an emperor and has the heart of the Buddha,” said Xiao Hui, a tea maker in Wuyishan, a misty riverside town in Fujian, southern China. She showed me the dark, tangled, unfinished-looking Da Hong Pao leaves from her family's tea gardens in Wuyishan. Xiao and her family, tea makers for many generations, still go into the mountains every spring to call on the tea god, Lu Yu, to bring new shoots.

Wuyishan's startling karst landscape has been famous for tea for centuries. The rain that pours down the limestone gorges and karst pinnacles, flooding the narrow mountain streams and tumbling waterfalls, is heavy with minerals that impart flavour. Today, every other shop in Wuyishan has a tea-tasting table set for the ritual of gong fu cha (kung fu tea) – the closest China comes to the Japanese tea ceremony –and shelves stacked with a gaudy selection of tea leaves.

Travelling to Wuyishan, I discovered that many Da Hong Pao teas are surprisingly affordable. Though aged or antique versions can sell for extremely high prices, a Da Hong Pao of reasonable quality can cost around $100 per kilo in Wuyishan. But every genuine Da Hong Pao originates with a cutting from a single group of mother trees. And it's these original trees that produce the rare and sought-after original tea.

“The original Da Hong Pao is so expensive because there are hardly any of the original tea trees left,” explained local tea master Xiangning Wu. “And antique versions are very valuable, almost priceless.” In fact, it’s all so exclusive that specialist brokers navigate the rarefied world of China's ultra-wealthy tea collectors, connecting those who need to sell with those who wish to buy.

But it's not just the Chinese who value Da Hong Pao. In 1849, British botanist Robert Fortune came to the Wuyishan mountains on a secret mission, part of the agro-industrial espionage at which the colonial East India Company excelled.

Britons were, then, as now, obsessed with tea, and China – from where the Brits also bought silk and porcelain ­– was the only place they could get it. But Britain made little that China wanted, creating a massive trade deficit. An obvious way of resolving the balance of trade was to do what the East India Company had done with other valuable plants: steal the seeds (or, better, cuttings) and grow them elsewhere. If Britain could make its own tea in India, the nation would be that much less dependent on China.

But Britain couldn't. The tea seeds that previous spies had sourced from Guangdong simply would not grow – and the native Indian tea bushes, a different type of plant to Chinese tea, just didn't taste right.

Enter Fortune. His aim was to track down China's best tea – Da Hong Pao – and to learn how to grow it. And since almost all of China was closed to foreigners on pain of death, disguise was essential. Fortune hired a servant, cut his hair, affixed a purchased pigtail and embarked for Wuyishan in search of Da Hong Pao.

Just as they do today, tea gardens clambered up and around the mountains, squeezed into the narrowest of gorges and perched on the steepest of slopes. And just like today, a handful of precious bushes balanced in a brick terrace in a vertiginous limestone face, with three Chinese characters carved in scarlet: Da Hong Pao. The name – Big Red Robe – references a scarlet blanket that a mythical emperor donated long ago in thanks for a miracle cure.

Fortune took up residence in the Tianxin Yongle Temple below Da Hong Pao, and – amid leisurely discussions as to whether shoots picked by monkeys or virgins made the best tea – the botanist acquired seeds, seedlings and the secrets of their cultivation. When they reached India, these seeds, merged with indigenous Indian tea, would form the beginnings of an industry now worth billions of dollars a year.

Or, as Zhe Dao, now abbot of Tianxin Yongle told me: “In the 19th Century, some plant hunter came and took the seeds. But he didn't know how to make the tea so he needed the masters to teach him how.”

Tianxin Yongle was founded in 827AD. In 1958, during the Mao era, the monks were forced out, taking their tea-making knowledge with them. When Zhe arrived from the ancient city of Suzhou in 1990, what little remained of the temple was home to peasants.

“Back then it was just me,” Zhe explained. “Now I have a lot of disciples, so five or six years ago we started to make tea.”

The original Da Hong Pao trees sat on temple land, but Zhe left their management to the government. Production was tightly controlled – the few hundred grams the trees yielded every year were reserved for the state – and until recently, the trees were under constant armed guard.

I walked past the monastery's vegetable gardens and up and along the narrow, winding mountain paths to the original Da Hong Pao.

The trees looked tired and spindly. Estimates of their age vary widely, although 350 years gels with Fortune's account. It was hard to imagine these straggly bushes bursting with new growth.

And it seems that they won't. On 1 May, soon after the tea harvest begins, a red carpet will be rolled out to mimic the emperor's gift. Beautiful women dressed in traditional costume will ascend the mossy steps and perform a ritual.

But there will be no harvest. These precious, ancient bushes, last harvested in 2005, will likely never make tea again. Which means the scattered few grams collectors are lovingly storing, drying them each year to mature their flavour, will be more valuable than ever before. Perhaps as expensive as diamonds, given time.

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