The grinder kicked up a few rocks, breaking most of them open in the process.
Most of the broken rocks did not strike me as interesting, but this one did.
4 cm wide:

A closer crop:

Closer yet:

I live on a plain formed by cataclysmic floods at the end of the last ice age. As a result, the rocks in my yard are a thoroughly mixed jumble plucked randomly from probably several hundred square miles. A fair number of them came from ancient lava beds to the northeast, and many of those contain empty voids formed by gas pockets when the lava erupted.
This particular rock, however, seems rather more interesting. It appears to have started life as lava with voids, but then many of the voids got completely filled in with some brown crystalline material and a few of them just accumulated a thin white deposit on one side.
If anybody knows more about this material, I would be grateful to learn.
In the meantime, it was another opportunity to practice using Magic Lantern. This was 21 frames at f/8 shot with StepSize 1 and 3 steps per frame, using a Canon EF 100 mm f/2.8L IS USM macro lens on a Canon T1i camera. 1/3 second exposure time with incandescent illumination. Just camera on tripod -- no tether or rails. Very simple setup!
--Rik