Variegated leaf
Place the leaves in boiling water for six minutes to soften the cells, and then in ethanol (alcohol). When the leaves are drained of colour, spread them out on a flat surface. Soak the leaves in iodine and the green parts will turn blue/black and the non green parts will stay the yellowish brown colour of iodine liquid. The green parts contain starch, which makes iodine turn blue/black. The blue/black stain is what shows starch is present. You could take "before & after" photos to prove the black areas were green at the start.