Those are not emblems, in the image below you can see that the emblems are located in the lower right part of the Item (in this case is a symlink/shortcut).
The Icons for the user folders usually have a little representation of what that folder is all about. Those folders are "special" and are different from the rest of the folders on your system. In this example the folder is Shared/Public stuff, next to this is a regular folder.
Icons are located in two locations:
Depending on the the icon theme, they will placed in different categories (most are in /places).
This will help you understand it better:
Why does, for example, Nautilus know that it has to apply a special icon for these directories? Is there some flag on the actual dir, a metadata field in GVFS or is this hard-coded into Nautilus?
It's a standard set by the freedesktop organization called xdg-user-dirs.
From the site:
xdg-user-dirs is a tool to help manage "well known" user directories like the desktop folder and the music folder. It also handles localization (i.e. translation) of the filenames.
The way it works is that xdg-user-dirs-update is run very early in the login phase.
This program reads a configuration file, and a set of default directories. It then creates localized versions of these directories in the users home directory and sets up a config file in $(XDG_CONFIG_HOME)/user-dirs.dirs (XDG_CONFIG_HOME defaults to ~/.config) that applications can read to find these directories.
You can read more about the XDG Base Directory Specification here.