This would not be that difficult,
marble = imread('https://upload.wikimedia.org/wikipedia/commons/f/f3/SierraXenolith.JPG');
imagesc(marble)
marble2 = marble(:,:,2)<120;
imagesc(marble2)
just by intensity you can distinguish the central part, if you use some morphological operators you could remove small parts and consolidate the centre. The only problem is the finger ...