Interesting

So, the stone used to construct Washington Monument changes type about 40 metres up and this is why...

"The unfinished Washington Monument, seen to the right in a Civil War period photograph, was not the capital landmark that it is today. Simplified from the original 1836 design by Robert Mills, the Washington Monument was not to be totally completed inside and out for twentyfive more years, in 1888"
https://www.whitehousehistory.org/photos...

The quarry used for the original stone had closed by then, so a different stone was used.