The 'core' icons
"address-book",
"adjust",
"align-left",
"ambulance",
"anchor",
"angle-double-down",
"angle-double-left",
"angle-double-right",
"angle-double-up",
"angle-down",
"angle-right",
"angle-up",
"archive",
"arrow-circle-o-right",
"arrow-down",
"arrow-left",
"arrow-right",
"arrow-up",
"arrows-alt-h",
"arrows-alt-v",
"asterisk",
"at",
"award",
"backward",
"ban",
"bars",
"bed",
"bell",
"bell-slash",
"birthday-cake",
"bold",
"book",
"book-reader",
"bookmark",
"bookmark-delete",
"briefcase",
"bullseye",
"calendar-alt",
"calendar-day",
"caret-down",
"caret-left",
"caret-right",
"caret-up",
"certificate",
"chart-bar",
"chart-pie",
"check",
"check-circle",
"check-square",
"chevron-circle-down",
"chevron-down",
"chevron-left",
"chevron-right",
"chevron-up",
"circle",
"clipboard",
"clock",
"cloud-upload-alt",
"code",
"cog",
"coins",
"columns",
"comment",
"comment-slash",
"comments",
"compress",
"copy",
"credit-card",
"crosshairs",
"crown",
"cube",
"desktop",
"discourse-amazon",
"discourse-bell-exclamation",
"discourse-bell-one",
"discourse-bell-slash",
"discourse-bookmark-clock",
"discourse-compress",
"discourse-dnd",
"discourse-emojis",
"discourse-expand",
"discourse-other-tab",
"discourse-sparkles",
"discourse-threads",
"download",
"ellipsis-h",
"ellipsis-v",
"envelope",
"envelope-square",
"exchange-alt",
"exclamation-circle",
"exclamation-triangle",
"external-link-alt",
"eye",
"fab-amazon",
"fab-android",
"fab-apple",
"fab-chrome",
"fab-discord",
"fab-discourse",
"fab-facebook",
"fab-facebook-square",
"fab-github",
"fab-instagram",
"fab-linkedin",
"fab-linux",
"fab-microsoft",
"fab-patreon",
"fab-twitter",
"fab-twitter-square",
"fab-wikipedia-w",
"fab-windows",
"far-bell",
"far-bell-slash",
"far-calendar-plus",
"far-chart-bar",
"far-check-square",
"far-circle",
"far-clipboard",
"far-clock",
"far-comment",
"far-comments",
"far-copyright",
"far-credit-card",
"far-dot-circle",
"far-edit",
"far-envelope",
"far-eye",
"far-eye-slash",
"far-file-alt",
"far-frown",
"far-heart",
"far-image",
"far-list-alt",
"far-meh",
"far-moon",
"far-smile",
"far-square",
"far-star",
"far-sun",
"far-thumbs-down",
"far-thumbs-up",
"far-trash-alt",
"fast-backward",
"fast-forward",
"file",
"file-alt",
"file-audio",
"file-csv",
"file-image",
"file-invoice",
"file-signature",
"file-upload",
"file-video",
"filter",
"flag",
"folder",
"folder-open",
"forward",
"gavel",
"gift",
"globe",
"globe-americas",
"grip-lines",
"group-plus",
"group-times",
"hand-point-right",
"hands-helping",
"heart",
"history",
"home",
"hourglass-start",
"id-card",
"image",
"images",
"inbox",
"info",
"info-circle",
"italic",
"key",
"keyboard",
"language",
"layer-group",
"link",
"list",
"list-ol",
"list-ul",
"lock",
"magic",
"map-marker-alt",
"microphone-slash",
"minus",
"minus-circle",
"mobile-alt",
"moon",
"paint-brush",
"palette",
"paper-plane",
"pause",
"pen-fancy",
"pencil-alt",
"play",
"plug",
"plus",
"plus-circle",
"plus-square",
"power-off",
"puzzle-piece",
"question",
"question-circle",
"quote-left",
"quote-right",
"random",
"redo",
"reply",
"robot",
"rocket",
"save",
"search",
"search-minus",
"search-plus",
"share",
"shield-alt",
"sign-in-alt",
"sign-out-alt",
"signal",
"sliders-h",
"sort",
"sort-alpha-down",
"sort-alpha-up",
"sort-numeric-down",
"sort-numeric-up",
"spell-check",
"spinner",
"square-full",
"stamp",
"star",
"step-backward",
"step-forward",
"sticky-note",
"stream",
"sync",
"sync-alt",
"table",
"tag",
"tags",
"tasks",
"th",
"thermometer-three-quarters",
"thumbs-down",
"thumbs-up",
"thumbtack",
"ticket-alt",
"times",
"times-circle",
"tippy-rounded-arrow",
"toggle-off",
"toggle-on",
"trash-alt",
"undo",
"unlink",
"unlock",
"unlock-alt",
"upload",
"user",
"user-check",
"user-cog",
"user-edit",
"user-friends",
"user-plus",
"user-secret",
"user-shield",
"user-times",
"users",
"wrench"
Well, theoretically at least… I have just tried to look for one and not found it, so perhaps it’s not 100%. I won’t tell you which so I don’t spoil the game. 
