(IDEX Online) - Forty per cent of diamond artisans have reportedly failed to return to work in Surat, India, as fears mount of a third wave of coronavirus.An estimated 400,000 migrant workers returned to their home villages for the Hindu festival of Holi, at the end of March, and have remained there, according to the Financial Express newspaper.It says production across the city's 6,000 manufacturing units - which produces over 90 per cent of the world's diamonds - is down by 20 per cent as a result. And it quotes Dinesh Navadiya, Gujurat chairman of the Gems and Jewellery Export Promotion Council, as saying that many are struggling to deliver goods on time.Health authorities in India have been warning of a third Covid wave, following the catastrophic second wave that peaked in early May.Migrant workers in sectors other than diamonds - primarily textiles - have also been staying away from big cities because of infection fears.Generic pic shows diamond polishing in Surat