Phu Nhuan Jewellery, Vietnam’s largest listed jeweller, is facing a crisis of confidence after police detained the former head of its gem certification subsidiary over his alleged links to a transnational diamond-smuggling ring. Shares in PNJ, as the company is known, have plummeted more than 25 per cent since news of the investigation broke in early July. Police accuse Dang Ngoc Thao, the former director of wholly owned unit PNJ Laboratory, of involvement in a criminal network that allegedly...