Sprockets::FileNotFound in ... ... couldn't find file 'material_icons' with type 'text/css'

Fix: this is how your app/assets/stylesheets/application.scss should look:

/*

• This is a manifest file that'll be compiled into application.css, which will include all the files
• listed below. *
• Any CSS and SCSS file within this directory, lib/assets/stylesheets, or any plugin's
• vendor/assets/stylesheets directory can be referenced here using a relative path. *
• You're free to add application-wide styles to this file and they'll appear at the bottom of the
• compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
• files in this directory. Styles in this file should be added after the last require_* statement.
• It is generally better to create a new file per style scope. * *= require_tree . *= require_self */

@import "materialize";
@import "https://fonts.googleapis.com/icon?family=Material\Icons";