Gabbro!

And pretty fresh at that, considering where we found it.

A cobble in the Aliakmon, miles and miles away from any source of fresh gabbro.

On this surface, the milky crystals are plagioclase cut at some angle that you can see their twinning, while one crystal (top center) does have visible twinning. The dark mass in the center is clinopyroxene, and actually looks better in real life, with visible cleavage reflecting when the light is held the right way.

The pyroxene cystal is about 1.5 cm scale, making this actually a gabbro pegmatite.