Albite

[colohank]

I was walking along, photographing birds at a wildlife area in Grand Junction, Colorado, when a strong flash of sunlight emanating from ground-level caught my eye. I traced it back to this rather unremarkable little cobble, which I collected and took home with me in hopes of duplicating the effect. The wildlife area is located on a gravel terrace at the confluence of the Colorado and Gunnison Rivers, where constituent gravels are a mixture of igneous, metamorphic, and sedimentary rocks from varied upstream sources in the Rockies and San Juans. This particular specimen contains conspicuous phenocrysts (large, included crystals) of albite, a sodium-rich feldspar.

A direct beam of light from one compact LED lamp did the trick. It's reflected by albite's characteristic polysynthetic twinning surfaces. Note that the source of light in this instance is located to the right of the rock, almost perpendicular to the axis of view, not an orientation one might ordinarily expect capable of producing such a bright reflection.

[MarkSturtevant]

That is interesting. I so very much admire the terminology used by geologists and ecologists, btw! Wish I could talk like that.
So that is some kind of crystal that is embedded in the rock?

[Pau]

So that is some kind of crystal that is embedded in the rock?

Well, this is a magmatic rock. The big well shaped crystals* were the fisrt ones to solidify with enough space and time, then the rest of the magma solidified at higher speed and the other minerals had less time and space and so they are smaller and maybe more irregular shaped.

Phenocrystals in geologists jargon

[colohank]

Probably a granite porphyry. The pits on the surface are where more resistant quartz grains have been plucked out by mechanical action, from countless impacts as the rock was tumbled and banged and bumped in the traction load of a flowing river.