National Mineral Development Corporation, NMDC, has unearthed a 37.68 carat diamond, the biggest ever gem produced from the diamond mines at Panna in Madhya Pradesh. The experts have valued this gem between 2.5 and 5 crore rupees. NMDC officials said, the white rough diamond in octahedron shape is of “good quality”.The mining major's Mumbai-based valuers will arrive in Panna sometime next month to assess the commercial value of the precious gem. The gem is expected to be auctioned later. The previous big haul from Panna mines was in June 2010 when a 34.37 carat diamond was recovered.