Aww yeah, Material-UI v4 is here!
Paper

Paper

In Material Design, the physical properties of paper are translated to the screen.

The background of an application resembles the flat, opaque texture of a sheet of paper, and an application’s behavior mimics paper’s ability to be re-sized, shuffled, and bound together in multiple sheets.

This is a sheet of paper.

Paper can be used to build surface or other elements for your application.