Under the 'Sawmill' section of the woodcutting guide, when you cut a plank with the crystal saw, it doubles the amount of xp you would receive when cutting the plank with a normal saw. Such as, with a normal saw when cutting a short or long plank you'd receive 15 xp, but, when using the crystal saw you would receive 30, just pointing out that it doesn't mention that you will get double, or extra xp when using the normal or crystal saw. :-D