We have been asked this question after the publication of the item "Why does wet sand look darker than dry sand?"

There are many beautiful beaches locally, with fine, white sand.

For some other beaches in Hong Kong, the sand may be coarser. It also looks darker.

When we discussed "Why does wet sand look darker than dry sand?", we talked about the change in the direction of light (i.e. scattering) when hitting the sand grains. However, the scattering angle is not much different between coarse and fine grains. No matter what the grain size is, a sand grain is made of silica (or silicon dioxide) or granite, which is mostly silica. So scattering cannot explain why coarse sand looks darker.

Rather, as you can easily imagine, it is because light is more likely absorbed by a large grain than a small grain. Hence coarse sand looks darker than fine sand.