grind media (1)high silica (2)good hardness (3)grind medium (4)competitive price(5)bearable ... made pebbles/Flint pebbles/Silica pebbles/Cobblestone/Natural pebbles2.Size:2-4cm,4-6cm,6-8cm,8-10cm,10-12cm,12 ... )COLOUR OF PRODUCT AFTER BURNINGGRAY AND WHITE7.Physical dataSpecific weight : 2,66Hardness(mohs): 8Porosity : 0,38 %Apparent density : 1,608.to ...