Meaning: A showy article is not necessarily valuable.
The 12th century French thelogian Alain de Lille wrote "Do not hold everything gold that shines like gold" and in 1553 Thomas Becon used in The relikes of Rome: All is not golde that glistereth.
By using this idea in The Merchant of Venice, 1596, Shakespeare made it well-known.
O hell! what have we here?
A carrion Death, within whose empty eye
There is a written scroll! I'll read the writing.
All that glitters is not gold;
Often have you heard that told:
Many a man his life hath sold
But my outside to behold:
Gilded tombs do worms enfold.
Had you been as wise as bold,
Young in limbs, in judgment old,
Your answer had not been inscroll'd:
Fare you well; your suit is cold.
Cold, indeed; and labour lost:
Then, farewell, heat, and welcome, frost!
Portia, adieu. I have too grieved a heart
To take a tedious leave: thus losers part.