$ gio info ~/Documents | grep symbolic-icon
  standard::symbolic-icon: folder-symbolic, folder
$ gio info ~/Pictures | grep symbolic-icon
  standard::symbolic-icon: folder-symbolic, folder
$ gio info ~/Downloads | grep symbolic-icon
  standard::symbolic-icon: folder-download-symbolic, folder-symbolic, folder-download, folder

$ ls -1 /usr/share/icons/Mint-Y/places/symbolic | grep folder
  folder-documents-symbolic.svg
  folder-download-symbolic.svg
  folder-music-symbolic.svg
  folder-pictures-symbolic.svg
  folder-publicshare-symbolic.svg
  folder-remote-symbolic.svg
  folder-saved-search-symbolic.svg
  folder-symbolic.svg
  folder-templates-symbolic.svg
  folder-videos-symbolic.svg

$ gio set ~/Documents standard::symbolic-icon folder-documents-symbolic
  gio: Setting attribute standard::symbolic-icon not supported
$ gio set ~/Pictures standard::symbolic-icon folder-pictures-symbolic
  gio: Setting attribute standard::symbolic-icon not supported

~/.config/user-dirs.dirs