Today the largest rough diamond to be discovered in a century goes under the hammer at Sotheby’s. At 1,109 carats, the uncut “Lesedi La Rona”, mined in Botswana, may well exceed the auctioneer’s $70m guide price. That can only enhance the image of an industry that is struggling to sell to the “millennial” generation. Young consumers increasingly shun the taint of conflict and exploitation, and middlemen have been hit as banks balk at gemstones’ untraceability. Laboratory-made gem diamonds, grown using a technique called chemical vapour deposition, are emerging as rivals; these near-perfect crystals bear none of the messy ethical implications of the mined stones. That irks Lucara and other listed diamond miners, who say they play by the rules and conscientiously support development in countries like Botswana. Breathtaking finds like Lesedi La Rona help them make their case. But the industry’s reputation still needs a polish.