Should he sell a piece of fine glass to a man who will sell it on the basis that it is a diamond?
Question

                My friend works with producing diamonds for a company, and he found a piece of glass of very fine quality, which is so fine that from the outside it looks very much like a diamond. He showed it to one of the jewellers to buy it from him, but my friend is very sure that this man will take it and sell it to someone on the basis that it is a piece of diamond. My friend wants to know whether it is permissible to sell it to him or is it haraam to sell it to him? Is it permissible to sell it to someone else on the basis that it is a piece of glass?.
Praise be to Allah.
    Firstly: 

    You did not explain to us how your friend found this piece 
    and where he found it? 

    If this piece of glass has a high value and its owner is 
    looking for it, and your friend found it in the company or on the street and 
    the like, then it is lost property and he must look for its owner for a 
    year. If he does not find him, then he may benefit from it after a year on 
    the condition that if its owner comes some day, he will give it to him -- if 
    he still has it -- or he will give him its value if he has sold it. 

    For more information on the rulings on lost property, please 
    see the answer to question number 
    5049. 

    Secondly: 

    There is nothing wrong with your companion selling this piece 
    on the basis that it is a piece of glass, but it is not permissible for him 
    to sell it to one who he thinks most likely will sell it on the basis that 
    it is a diamond and will deceive and cheat the purchaser, because that is 
    cooperating in sin and transgression, which Allah has forbidden in the verse 
    (interpretation of the meaning):

    “but do not help one another in sin and transgression”

    [al-Maa’idah 5:2].

    The scholars of the Standing Committee for Issuing Fatwas 
    said: 

    Everything that will be used for haraam purposes or it is 
    thought most likely that it will be used in that manner, it is haraam to 
    make it, import it, sell it and distribute it among the Muslims. End quote. 

    Fataawa al-Lajnah al-Daa’imah, 13/109.