OK, I think I figured out this tweet. It's a lame attempt at "Gallows Humor" in my opinion.
On May 3, 2017 the following dialogue between Al Franken and James Comey took place…
FRANKEN: Thank you to the ranking member and I admire your hanging in there and being made of stone was it?
COMEY: Sandstone I think.
There is no other mention of Sandstone in the transcript.
https://cc.bingj.com/cache.aspx?q=%22sandstone%22+comey+&d=4672516548003741&mkt=en-US&setlang=en-https://cc.bingj.com/cache.aspx?q=%22sandstone%22+comey+&d=4672516548003741&mkt=en-US&setlang=en-US&w=RSV1NR1tz3VM0K0M8hG8dxDYIhFETaUnUS&w=RSV1NR1tz3VM0K0M8hG8dxDYIhFETaUn
On April 9, 2010 Al Franken visits Sandstone Federal Prison
http://www.citypages.com/news/al-franken-lets-the-cowbell-rip-on-video-6553067
Sandstone Federal Prison has a "Snitch House" that houses high security inmates who have informed on others and would be in jeopardy among the general population.
The Federal Correctional Institution, Sandstone (FCI Sandstone) is a low-security United States federal prison for male offenders in Sandstone, Minnesota. It is operated by the Federal Bureau of Prisons (BoP), a division of the United States Department of Justice.
Not generally acknowledged by the BoP, FCI Sandstone also includes a separate high security wing complete with electric fences and gun towers. This unit, informally known as the "snitch unit" houses high security inmates who have informed on others and would be in jeopardy among the general population. The inmates in this unit are completely segregated from the other Sandstone inmates, although they are served by the same staff.
https://en.wikipedia.org/wiki/Federal_Correctional_Institution,_Sandstone