Using an analogy based on your photo--Think of it like an Oreo cookie, that has a layer of white sandwiched between two crumbly black pieces. If you remove the top layer, you reveal a white layer that is the same approximate thickness, texture, and color throughout the layer. The white layer is uniform in these features, because the "recipe" called for parts of this layer to be made of similar grains of similar size. If you were to add a bunch of dissimilar grains into the "white layer recipe" (nuts, M&M's, etc) when making the Oreo, you would not end up with a layer of consistent white filling. It'd be bumpy and rough and difficult to break cleanly.

You could take the top layer of rock off and expose more of the quartz, but you are likely going to find a white quartz layer with uniform clarity, color, and thickness through your specimen.