grains which have undergone, on a small area of their surface, an obvious change in their natural colour ; the stains may be of different colours (blackish, reddish, brown) ; deep black striations are also to be regarded as stains. If the colour of the stains is sufficiently marked (black, pink, reddish-brown) to be immediately visible and if they cover an area not less than half that of the grain, the grains must be considered to be yellow grains