Possibly could be staining caused by long history of use - organics etc leaching into underlying soil from a house site. However the site was dug in hot dry summer conditions and the darker soil could simply be a fresher area more recently cleaned up by the excavators (look at all the slots)

The polishing was done as part of the original finish but there is evidence for cutting edges being reground later too, sometimes after breaking. Or alternatively sometimes not used at all, just as an object of beauty and meaning

Thanks so much, it's lovely to feel appreciated 👍😊