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.