Why are ice cubes white towards the centre but transparent towards the edges?

I’ve just finished a long work week, got home, and made a gin and tonic, and I’m looking at the ice cubes bobbing in it and noticed something I’d not paid attention to before: the cubes seem to be opaque in their core but more-transparent towards their crust. You can see what I mean in this photo.

Now other photos, like this one, show that the effect isn’t universal (and I’m pretty sure I’ve seen perfectly clear ice cubes before, too). So… why are mine opaque?

If it helps:

• Mine were made in a conventional food freezer at -20°C
• I’m in a hard water area
• I’m using an ice cube tray which makes “cubes” that are actually trapezoidal prisms

Thanks!