Your eye is a diamond, someone else's is glass
This is what they say when it is more important for a person to see everything for himself than to believe the words of others. Even if the witness is honest and big-eyed, it's still not what you see in person.
The phrase was born in the old days, when the diamond was a symbol of clarity and value, and the glass was a cheap imitation. Your vision is like a jewel, and someone else's vision, though transparent, is not real.
It's true: no matter how much they tell you, the eye itself will do everything right.
Proverbs
๐ฟ Russian proverbs