"Lustrous as ice on a sunbeam," this pigmented glass can be used structural applications or repurposed for a variety of projects. Over time "vitrolite" became a generic term for structural glass, but the originator is identified by the fine, parallel grooves on its back (to maximize grip).
This page has some great background and history on the material, with foucs on its use in the DC area.
Check out other items on this job!