all that glitters is not gold
Definition from Wiktionary, the free dictionary
Alternative forms 
Translation of Latin nōn omne quod nitet aurum est (“not all that shines is gold”).
- Things that appear valuable or worthwhile might not actually be so, things that look nice might not be as good as they look.
Related terms 
things that look nice might not be as good as they look
- Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, ISBN 0-679-44554-4, p. 9.