All that glitters is not gold.

“All that glitters is not gold.”

Appearances can be deceiving; not everything that looks valuable or promising truly is.

— Traditional United Kingdom Wisdom, United Kingdom

Source: English proverb traced to Chaucer's The Canon's Yeoman's Tale (14th century), popularized as 'All that glisters is not gold' in Shakespeare's The Merchant of Venice (1596)