Ever since the discovery of a huge diamond pipe in its sun-parched interior, Botswana has enjoyed peace and prosperity to an extent unknown to its neighbours. Last week its red earth produced the biggest diamond found anywhere in more than a century - a 1111-carat wonder the size of a tennis ball.But the country's future might now be not so glittering.Botswana's mines are expected to run out of gems - the sale of which makes up 80% of export earnings - in as little as 15 years.Already President Ian Khama is facing questions about what will sustain the country, one of the few in Africa to reach middle-income status, after mining.If Khama is unable to come up with a good alternative, his Botswana Democratic Party faces eviction from power at the next poll in 2019.But this week senior politicians and De Beers convened a conference in the capital Gaborone to try to map a way forward.Landlocked, Botswana is poorly positioned to become a manufacturing hub, and droughts have caused water shortages and power cuts.Bruce Cleaver, De Beers' strategy officer, said the company would help as much as was "practicable"."We can't pretend to have all the answers, and neither do we think it's our role to solve all these problems, but we do think it's a useful debate to start having about what happens to Botswana when the diamonds are gone."..