The last time I showcased the mineral Bornite, I found it in a general shale pile. This time, while working some limestone locally there is a shale shelf below the limestone that I have carved flat with a shovel. Several of the exposed pieces of shale in that particular piece has an odd shine to it when viewed with an external light source. I took a few pieces and ended up with the specimen below.
The structure on this is quite fascinating with a pitted appearance crossed with some long crystal type structures in the middle and top of the photo. This image was 5 exposures taken through a microscope lens and focus stacked.
Bornite is 5 Copper atoms joined with 4 Sulfer atoms with a single Iron atom. The Chemical Formula is: Cu5FeS4. This repeats in a pattern. Pyrite, another common mineral is FeS2. Bornite is primary a Copper ore, which is mined when quantities are high enough for profitable recovery.