So what does fortune do:
Increases block dropsFor coal, diamond, emerald, nether quartz, and lapis lazuli, level I gives a 33% chance to multiply drops by 2 (averaging 33% increase), level II gives a chance to multiply drops by 2 or 3 (25% chance each, averaging 75% increase), and level III gives a chance to multiply drops by 2, 3, or 4 (20% chance each, averaging 120% increase). 1 drop has a weight of 2, and each number of extra drops has a weight of 1.
For redstone, carrots, glowstone, sea lanterns, melons, nether wart,potatoes, and wheat (seeds only), each level increases the drop maximum by +1 (maximum 4 for glowstone, 5 for sea lanterns, and 9 for melons). Fortall grass, each level increases the drop maximum by +2.
Fortune increases the probability of flint dropping from gravel, and saplingsdropping from leaves, and apples dropping from oak and dark oak leaves.