Here's what I did to find it.
I did a straight-forward search on [ Newseum stone ] which led me to this article in KnoxvilleBiz.com
"...The marble came from the Endsley Quarry in Friendsville and was provided by Tennessee Valley Marble before the company was purchased last year by Tennessee Marble Co.
Architects chose Tennessee pink marble for the tablet because the same stone was used on the exterior of the National Gallery of Art, which is across the street, and in several other prominent buildings and monuments throughout the city..."
If you look to the right (east) of the quarry, you can see a stone cutting facility that you can't see from the road. But if you zoom in, you can spot large blocks of stone sitting around outside, looking like small cars.