Why is streak different from color? Many minerals have a streak that's surprisingly different from it's outward color. Why is this? What causes the optical properties to change if the mineral is powdered?

Thanks!
It mostly has to do with the fact many minerals are partially translucent. 

Trace impurities or even crystal structure can dominate the color of a translucent material but when ground in to a fine powder (streak) they can no longe do so and you can see the true color. 

In other minerals surface oxidation can mask the true color.