Fracture planes, that is. Here's a low-res, Sketchbook-edited version of a photo I took earlier this winter. I added an inset offering a closeup of the detail:
In the obscure niche that is ice-photography, reflection is as important as refraction, and fracture planes can contribute a lot to the former.
A large chunk of ice always fractures with a loud crack when its surface temperature rises suddenly, but the fracture plane differs. From what I can tell the variables include the temperature gradients, the thickness of the ice slab, the chemistry of the water, and the air temperature at which the ice formed.
Most of the fracture planes I see are striated and rough like the one above. But sometimes the fracture has a checkerboard pattern:
More rarely the fracture planes are as smooth as a half-silvered mirror, but with unique and random details such as the yellow pattern below: