One day, a young pupil asked his teacher, "Sir, I don't understand why you wear so ordinary clothes. Your clothes don't show that you are a very learned teacher, who does a great job of imparting knowledge to hundreds of pupils."
Removing his diamond ring, the teacher gave it to him and told smilingly, "I will definitely satisfy your curiosity. First, you go to the market and sell the ring to a vegetable vendor or any other shopkeeper for at least one gold coin."
The young man went to the market and came back with the ring after some hours. He told, "Sir, I have tried to sell the ring to a vegetable vendor, a general merchant and some shopkeepers. But no one is willing to buy it for one gold coin."
The teacher said, "Ok, now you go to a jeweler's shop and try to sell it."
The young man did as told. He came back after some time and told the teacher, "Sir, no jeweler is willing to buy it for one gold coin. All jewelers to whom I went to sell it off told, 'The ring is so costly that all the jewelers of the town even together cannot purchase it. This ring is priceless.'"
The teacher explained, "As one cannot have an idea about the cost of the ring only by looking at it, similarly one cannot have an idea about a person only by looking at his dress. One has to assess deeply the person to know well about him or her as the jewelers did with the ring. We shouldn't judge a person only by the outer appearance as it may be quite deceptive."