Silver prices tumbled to a three-week low by losing Rs 4,900 to Rs 90,900 per kg, while gold slipped Rs 100 to Rs 78,700 per 10 grams in the national capital on Thursday on frantic selling by stockists and retailers, according to the All India Sarafa Association. The yellow metal of 99.5 per cent purity declined Rs 400 to Rs 78,300 per 10 grams on Thursday.