Identification of what my wife believes isn't a rock

[PennyPong]

While planting a rose in our somewhat rocky soil, I encountered an interesting rock. My wife was not convinced that it is, in fact, a rock because of the shape. So, to my horror, she banged it a couple of times on another, larger rock but fortunately did not break it.

It has a diamond-shaped cross section, as shown in the photos, and is lustrous on two opposing sides while dull on the other two (opposing) sides. It is not noticeably magnetic, nor does it attract magnets.

In the first two photos, I wet the rock and shone a bright light on it to bring out the color.

Anyone know what this is? "Yep, it's a rock all right" will be helpful in ending an argument, but I am also interested in what kind of rock it is.

 

[north1]

Possibly a Green Hornfel rock. It is a metamorphic rock formed from intense heat and pressure. Common protoliths that can produce green hornfels include sedimentary rocks such as shale and sandstone, as well as igneous rocks like basalt and granite.

 

[Odanscoils]

Is it heavy ? Magnetic ?
If it's stone. Definitely looks hand shaped.
I'd swear I see chipping or flinting marks.

What it's supposed to be I haven't a clue.

Possibly hit an end with a grinder.

I've had grinder facing stones look like that.

Used to reface the grinder wheel.

 

[PennyPong]

A relative did quite a bit of research on my behalf. She suggested that it could be columnar basalt formed by volcanic action. I did some reading up on this and I believe this is a strong candidate, if not the actual ID. This also convinced the wife that it is an actual rock and not construction debris.

It seems that columnar basalt is formed in the presence of quartz, and often takes on a hexagonal shape, but sides as few as three and as many as 15 have been discovered. Columnar basalt is also the only candidate that has a basis for the shape, which is what has always intrigued me.

I defer to the wisdom and experience of the elders here (as I do with the She Who Must Be Obeyed, this particular incident notwithstanding), and am open to other suggestions. But for the moment, I like columnar basalt the best.