Definition
This idiom suggests that appearances can be deceptive; something that looks attractive or valuable may not actually be so upon closer inspection.
The restaurant looked great, but the food was actually quite mediocre; it just goes to show that all that glitters is not gold.
His new car looked luxurious, but it had many problems; it goes to show that all that glitters is not gold.
That investment opportunity looked promising, but after careful study, it turned out to be very risky; it truly proves that all that glitters is not gold.