The Surat Diamond Bourse — billed as the world's largest office complex, surpassing even the Pentagon in size and symbolizing India's growing trade ambitions — stands eerily quiet, with only a few traders at work.

The reason: business is sluggish, and the outlook is grim.

Exports by India's diamond industry have already hit a two-decade low on weak Chinese demand, and now higher tariffs under U.S. President Donald Trump threaten to sever access to its largest market, which accounts for nearly a third of its $28.5 billion annual shipments of gems and jewelry.