Wiktionary
prov. Things that appear valuable or worthwhile might not actually be so; things that look nice might not be as good as they look.
Wikipedia
All that glitters is not gold is a well-known saying, meaning that not everything that looks precious or true turns out to be so. This can apply to people, places, or things that promise to be more than they really are. The expression, in various forms, originated in or before the 12th century and may date back to Aesop. The Latin is Non omne quod nitet aurum est.
Chaucer gave two early versions in English: "But al thyng which that shyneth as the gold / Nis nat gold, as that I have herd it told" in " The Canon's Yeoman's Tale", and "Hyt is not al golde that glareth" in " The House of Fame".
The popular form of the expression is a derivative of a line in William Shakespeare's play The Merchant of Venice, which employs the word "glisters," a 17th-century synonym for "glitters." The line comes from a secondary plot of the play, the puzzle of Portia's boxes (Act II - Scene VII - Prince of Morocco):
Panning for gold often results in finding pyrite, nicknamed fool's gold, which reflects substantially more light than authentic gold does. Gold in its raw form appears dull and does not glitter. The expression is also found in Yiddish (nit als vos glanst iz gold), especially amongst Hasidim, and also appears in a Hebrew work of Mendele Mocher Sforim.
In pop culture, this phrase shows up in Bob Marley's " Get Up, Stand Up". It is also used as lyrics in the song "A Guided Masquerade" by Alesana, in the song "Domino Rain" by Antemasque, in the song "Gold" by Prince, and in the Kanye West song, " Family Business".
"Not all that glitters is gold" is an alternative formulation.
Another common formulation with the same meaning is "All that shines is not gold", as seen in the title and refrain of the song "All That Shines Is Not Gold" and in the lyrics of " Next Time You See Me" as well as Curtis Mayfield's " That's What Mama Say".
The inverse of this expression, "All that glitters is gold," is a lyric in the Led Zeppelin song, " Stairway to Heaven", the Smash Mouth song, "All Star", the Death in Vegas song, "All That Glitters", and in the Future Islands song "A Dream of You and Me" followed by the lyric "Don't believe what you've been told".
Usage examples of "all that glitters is not gold".
I have heard say that 'behind the cross there's the devil,' and that 'all that glitters is not gold,' and that from among the oxen, and the ploughs, and the yokes, Wamba the husbandman was taken to be made King of Spain, and from among brocades, and pleasures, and riches, Roderick was taken to be devoured by adders, if the verses of the old ballads don't lie.