Crushed pebbles shone differently when viewed from different angles

Question

I was roaming in a field. Crushed and pulverized pebbles were scattered in a corner of the field. From where I stood, I could see that some of the pebbles shone extraordinarily in the sunlight, while others remained ordinarily lit. I changed my angle of vision, and some of the ordinarily lit pebbles now began to shine in the extraordinary fashion, while all of the extraordinarily lit pebbles mentioned before became ordinarily lit.

Each time I viewed the pebbles from a different angle, the same thing happened again and again. Why would it happen?

Answer 1

Most likely this is because the randomly oriented pulverised pebbles have flat surfaces reflecting the sun. Those with surfaces oriented correctly to reflect the sunlight to your eyes wil appear brightest. If you move, a different angle of reflection will be required, and the set of pebbles that is oriented will change.

Answer 2

You may be describing a form of retroreflection in which a surface seems particularly bright when the light source is (almost) directly behind the observer:

If this is happening, then you would see a much brighter reflection from areas that are 180° from a bright light source (like the sun), and as you move around, the area with favorable reflection would also change. The phenomenon is called "Heiligenschein" - German for "holy halo", because in some circumstances if this is happening when you look at your own shadow it can look like you have a halo around your head.

There are several possible mechanisms for retroreflection. The best known is the corner cube - this is not likely to be the source of your accidental retroreflector, though. More likely is the presence of little beads of moisture: these beads focus the light (of the sun) onto the surface behind them; and light reflected from these surfaces is focused by the same beads of moisture, and back towards the light source. When the observer is in line with the light source, you will see a significantly brighter source than a simple Lambertian reflection - things will appear much brighter. The phenomenon is illustrated in this diagram from Alistair B. Fraser, "The sylvanshine: retroreflection from dew-covered trees," Appl. Opt. 33, 4539-4547 (1994)

This particular illustration is for a dew drop on a leaf - but the same principle applies for a drop of liquid on any surface. Wikiversity has a nice example observed from a hot air balloon:

There is one other explanation - although it's much less pronounced, I have observed this when flying over vegetation. When you look down towards the shadow of your plane, you tend to see a bright halo right around the shadow. This corresponds to the region where the shadow cast by the sun is directly behind the object you observe - so everything you see is in "bright sunshine" with no shadows, making things a little bit brighter. Depending on the height of the vegetation, the size of this halo will change a little bit. This could be called "shadow hiding in the shadow" - as was done in this Tweet:

In principle pebbles or other rough surfaces could have the same effect - but it's likely to be less pronounced than the retroreflection mechanisms I described above.