This effect is quite common. Take any solid chunk of mostly transparent material, introduce small structures that create little pockets of air, and you end up with a white material. For example, a solid block of ice is clear, but ice formed into microscopic structures (snow) is white. A solid block of salt (halite) is transparent, but salt ground down to a pile of grains (table salt) is white. A solid block of quartz is clear, but quartz filled with air bubbles (milky quartz) is white, and quartz ground down to a pile of grains (sand) is white. A cup full of water is clear, but a collection of tiny liquid water droplets (a cloud) is white. In each case, it is the existence of a microscopic material structure filled with tiny air pockets that leads to multiple reflecting surfaces, and therefore to strong reflection.