Gujarat Amuja Cements Ltd, India’s third-biggest cement maker by capacity, reported on Thursday a 50 per cent jump in quarterly earnings, buoyed by strong shipments and firm prices.   Gujarat Amuja, which owns a near 14 per cent stake in rival Associated Cement Companies Ltd, said net profit in its fiscal second-quarter ended December rose to 895.5 million rupees ($20.4m) from 595.4 million.   A Reuters poll had forecast the company’s net profit to grow by 25 percent from a year ago to a median 745 million rupees.   Turnover grew 42 percent to 6.19 billion rupees, compared with Reuters polls estimate of 6.28 billion rupees.