Minecraft wiki entry for Creating_a_challenge_map, associated text: Hide some little rewards for the player. Extra surprises make a challenge map exciting and fun. Diamonds or emeralds work well, because it can be a fun way to keep score. If you want a surprise ending for the players to build up to (literally and figuratively), make the maximum bonus count a multiple of 9, only if the prizes are diamonds, gold, lapis, emeralds, or iron (maybe even coal!) to make the necessary block; for others, it may vary.