With the 'efficiencyblocks' attribute you can set which blocks you can mine faster with the item (e.g: axes mines wood faster, pickaxes stone).
Valid values are integers between 0 and 255 (including both), 'all', 'none', 'pickaxe', 'axe', 'shovel'. You can set multiple values by separate them with a ','. The blocks you set for 'efficiencyblocks' have also to be in 'harvestblocks' in order to work.
Default value: 'none'
efficiencyblocks="all";
efficiencyblocks="1,2,3";
efficiencyblocks="pickaxe,1,5";