(IDEX Online) - More than 500,000 diamond workers are threatening strike action in Surat, India, to demand an end to "unfair" taxation.They're planning three days of walkouts and closures from 15 March in the world's cutting and polishing capital, according to the Times of India.Last year 13,000 diamond workers in the city were laid off because of weak market conditions.The Surat Ratnakalakar Sangh, an association of diamond polishers, said production had been slashed in recent months and workers were being further hit by a professional tax imposed by the Gujarat state government.The association's president, Jaysukh Gajera, said: "About 5.5 lakh (550,000) workers will stay away from work on March 16 to demand the abolition of professional tax. We are appealing unit owners to support workers by keeping their units shut for a day."