Why is this the answer?

The answer is coal because this clever riddle describes the life and job of this important black rock that comes from deep in the ground! First, the riddle says, "Black we are and much admired." If you have ever seen a piece of coal, you know it is very, very black, like a smooth, dark stone. Long ago, and even in some places today, people loved finding coal because it was a powerful source of energy. It was admired as a treasure because it could make life so much better. Then the riddle says, "Many seek us if they are tired." Think about a cold winter night. When you are tired and chilly, you want to be warm and cozy! People would use coal to start fires in their stoves or fireplaces. The coal would burn for a very long time and give off lots of heat. So, when they were "tired" of the cold, they would "seek" the coal for comfort and warmth. The middle line is a bit tricky: "We tire the horse, and comfort man." Imagine a huge, heavy train in the old days. It was too big for a real horse to pull! Coal was the fuel for the steam engine that pulled the train. The power from the coal could do the work that would have made a horse very tired. By taking over the really hard work, the coal comforted the man who would have been steering the horse. It provided the energy to keep the world moving and people warm. The last part gives us the final secret: "and turn White when we've fulfilled your plan." When you burn coal in a fire, it changes its color completely. It starts out black, but once it is all finished burning and has given you all of its heat, it leaves behind a light, powdery pile of white or gray ash. When you see the white ash, you know the coal's mission, or "plan," is fulfilled. It is all done! Every clue fits perfectly, showing why the correct answer is coal.