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After four years of tense negotiations, the world’s biggest diamond miner and biggest producer by value last month struck a landmark deal to guarantee supplies of the precious stones to global jewelers and retailers for years to come.

10-year sales contract and 25-year license awarded to De Beers mining in Botswana keeps a 54-year-old partnership alive. But critics say the new conditions, which are much less favorable to society, will eventually turn against the African nation.

President Mokgweetsi Masisi has led a populist campaign against De Beers with an eye on next year’s election, pushing it to give the country a bigger share of production in another blow to the company’s one-time monopoly and continued stewardship of the $16.5 billion industry.

“The deal is not good for De Beers, the industry and even Botswana in the long term,” said Richard Chetwode, an independent diamond sector consultant.

The previous deal, struck in 2011, was widely seen as one of the fairest in the mining industry, with Botswana receiving more than 80 percent of the value of the country’s diamond production when taxes, dividends and royalties paid by De Beers were taken into account.

The new deal gives Botswana more raw rock mined from its soil, with the former 25 percent share of the country’s output initially rising to 30 percent, then 40 percent in five years, and 50 percent by the time the deal expires in 2033.

Some observers warn that this will leave the miner with less money to pump into marketing, which is seen as vital to sales and the health of the diamond industry, exacerbating the decline in profits already expected from the deal.

Analysts at Morgan Stanley predict the arrangement will save $100 million a year initially and grow to $200 million, or 15 percent. De BeersIt predicts average annual core earnings of $1.3 billion over the next 10 years.

Other analysts are reserving judgment until more details are provided by De Beers, which has been 85 percent owned by Anglo American since 2012.

Al Cook, appointed chief executive of De Beers five months ago, highlighted the merits of the deal, insisting it would allow the company to lead diamond industry for the next 50 years, if not 100.

As recently as the turn of the century, De Beers controlled about 80 percent of rough diamonds, but that has fallen to 37 percent as Russia’s Alrosa — now the target of U.S. sanctions — has become an industry giant, according to industry analyst Edahn Golan.

“We knew from the start that without Botswana winning, De Beers wouldn’t have won,” Cook said. “In principle, this deal absolutely fulfills those ambitions.”

But he admitted the company needed to diversify beyond the African nation that supplies 70 percent of the group’s diamonds, saying De Beers needed to be “in more countries,” highlighting campaigns to explore new deposits in Canada, South Africa and Angola.

Kieron Hodgson, an analyst at Panmure Gordon, said an important strength of the deal was to avoid the huge disruption to global diamond supplies that would have occurred if the parties did not reach an agreement.

“They are Botswana diamonds. De Beers is only a tenant,” he said, while acknowledging that “if the state takes a higher percentage of the total wealth created, then that can be clearly negative for some stakeholders.”

Some warn that any drop in marketing spending from De Beers could hit sales and incomes across the sector and Botswana, especially as natural diamonds are threatened by lab-grown stones. The company’s post-war “Diamond is Forever” advertising campaign is credited with almost single-handedly creating a multi-billion dollar market.

Earnings for Botswana, which holds a 15 percent stake in De Beers, could also fall due to lower dividend payouts if sales fall.

Diamonds have helped Botswana, unlike many of its poorer neighbors, climb into the ranks of middle-income countries, with average earnings per person between $3.11 and $37.93 a day.

The country is even targeting high-income status above that upper limit around the mid-2030s when it begins underground mining at Jwaneng, the world’s richest diamond mine whose surface operations are nearly exhausted, as part of a multibillion-dollar investment plan to extend the project’s life beyond 2050.

Money generated by De Beers will also be used by Botswana to start a diamond development fund that will help fund investment in other sectors of the economy to create jobs, which is crucial in a country with an unemployment rate of around 20 percent.

A key challenge for Botswana will be achieving sales targets.

In its previous sales agreement with De Beers, it established the state-owned Okavango Diamond Company in 2011 to sell its allocated diamonds to international buyers through auctions. But more than a decade after its inception, ODC still struggles to sell large volumes.

James Campbell, a former De Beers employee who runs London-listed exploration company Botswana Diamonds, expects the ODC to move away from auctions and create a De Beers-like sales system to boost sales. This includes selling plots to a coveted list of De Beers customers, known as ‘observers’, at 10 annual events.

The small initial increase in production share that Botswana has secured reflects that “the state entity does not have the capacity to take 50 percent right away,” said Sheila Khama, former De Beers Botswana CEO and natural resources policy adviser.

There are also fears that Botswana will want to review the terms of the deal later and demand control over the sale of more than the agreed 50 percent of production before it expires in 10 years.

“The last deal in 2011 was seen at the time as a big win for Botswana,” said Hans Merket, a diamond specialist at the International Peace Information Service. “Yet 10 years later it was called a bad deal.