Belton House

Absolutely fantastic!

The water must be part of the answer maybe it was algae on the steps, the house is closed for the winter now so I will have to wait until the Spring to try on a dry day, it didn't occur to me to try a different pane of glass

I think it has something to do with the filter and the treatment of the stairs, maybe some oily cleaning or treatment liquid. Reminds me of the Diesel artefacts you see sometimes on wet streets, mainly from some machines and a lot less nowadays as things don't leak as much. The filter and the glass might have enhenced it for the camera. I don't think it's only the glass it would be the whole picture then.

For me, I will go with a reflection of the old glass pane (I have some in the house and I noticed that sometimes they work as prisms because of their uneven surface) that your filter picked up. If you look at the puddles in the middle of the gravel circle, they seem to reflect the whitewashed wall of the building, probably above (or rather below?) the window from which you took the shot. The fact that the rainbow like reflection ends sharply where the steps end, I think is due to the fact that the wet steps are a reflective surface, while the gravel is not and by the time you get to the equally reflective puddles, they no longer reflect the window, but only the white wall.
Very interesting as well as beautiful, Peter!