I am attempting to restore my 1891 Queen Anne Victorian in Cambridge, Massachusetts. There is a narrow (7") band of hard gray material encircling the entire stairway leading up to the recessed front door. I had initially assumed that this material was gray marble or granite, but there are some small areas of delamination which reveal a friable white crystalline substrate beneath the color. These crystals seem shiny rather than matte (and are especially white) and therefore do not resemble the lime putty (+sand and horsehair) plaster that we have inside the house. The colored layer is perhaps 1/8" or thinner and is rough and brittle.
Is this material scagliola? Or perhaps stucco lustro? Or Venetian plaster? What other possibilities should I consider (and rule out) before I proceed?
Unfortunately, this material was painted-over decades ago. Much of the paint is chipping off cleanly and does not appear to be taking off any of the original material with it, but in other areas the paint remains (relatively) firmly attached. It seems to me that paint removers (either organic solvents or caustic chemicals) would likely harm the surface of scagliola.
Can anyone offer any advice? I can't afford to hire an expert to perform a professional restoration, so I am doing all the work myself. What should I use to stabilize and/or repair the delaminated areas?
Finally, I have one last question: is it unusual to see scagliola outdoors? I suspect for other reasons that my home was originally built with a door and sidelights at the foot of the stairs, creating a sort of foyer, and it seems to me that the presence of scagliola (or whatever this is) would tend to lend further support to my hypothesis.
Thank you so much to anyone patient enough to read all the way through this overly-long message! 
~~Karen
