I sell diamonds retail. Currently, one can manufacture diamonds which look exactly like real diamonds. Is there anything wrong if I sell the manufactured diamonds for the same price as people charge for natural diamonds since the buyer isn’t losing anything? He wants his wife to look good and in terms of looks they are the same.
For a number of reason it is prohibited and any sale can even be voided by the customer.
The first issue is geneivas da’as. The Gemara (Chulin 94A) writes that one may not sell or give a gentile meat where the gid hanoshe has not been removed, if the gentile will think that it was removed, since that is geneivas da’as. The reason it is geneivas da’as (See Rashi) is because the gentile will feel gratitude towards the Jew for having given him meat which the Jew processed for his own use and then gave to the gentile. Even though the gentile just wants to eat the meat and as far as eating meat is concerned for a gentile it makes no difference if the gid hanoshe was removed or not, nevertheless it is classified as geneivas da’as.
Similarly in your situation, even though a lady wears a diamond for the looks, nevertheless if she is given the impression that she is receiving a natural diamond and it is really synthetic, you will violate the issur of geneivas da’as.
It should be noted that even if you charge the price of synthetic diamonds but give the impression that the diamond is natural you will still violate the issur of geneivas da’as. The reason again is because the customer will feel gratitude towards you for giving such a good price for a natural diamond, when you did not really give a reduced price.
It is important to note further that geneivas da’as is a serious issue and an issur de’Oraiso according to many Rishonim. Rav Eliashiv zatsal wrote (Kovetz Teshuvos) that one should consider this opinion authoritative.
The second issue is ono’o. One is not allowed to overcharge. Since in truth what is being sold is a synthetic diamond one must charge a price which is in line with the market price for synthetic diamonds. The rules of ono’o are that if the difference between the market price and the price which was paid is less than one sixth then the sale is valid and one need not return any money. If the difference is exactly one sixth then the one who overcharged must return the difference in price, and if it is more than one sixth the entire sale can be voided if the customer requests that.
Many Rishonim (See Rashi Kiddushin 42B) explain that the reason why one need not rectify anything if the difference is less than one sixth is that it is very difficult to determine an exact price. Therefore, Chazal gave a person some leeway in setting prices. However, even in this situation there are Rishonim such as the Rosh (his opinion is recorded in Choshen Mishpot 227, 6) who have a doubt that perhaps one who overcharges even by less than one sixth does violate the Torah prohibition to overcharge, and the opinion of the Ramban (Commentary to Behar) is that one unequivocally does violate the prohibition. If the amount of the overcharge is one sixth or more the seller certainly violates a Torah prohibition of ono’o.
Furthermore,it would seem that in this case according to all opinions, and even if there were no difference in price between the two types of diamonds, that the sale can be invalidated by either party on the grounds that it constitutes a mekach to’us. The Mishna (Bava Basro 83B) rules that in case a seller contracted to sell one type of object but gave a different type, then either party may invalidate the sale.
The Mishna cites as an example of a contract that may be invalidated by either party, a contract to sell white wheat where the seller delivered red wheat. Since white and red wheat are considered to be two different types, we view the delivery of the wrong type as non-compliance with their agreement. The reason white and red wheat are considered two different types is because they are not used interchangeably. Similarly, natural and synthetic diamonds are not interchangeable and are considered two different types.
In conclusion, if you give a customer synthetic diamonds instead of natural diamonds you will probably violate two serious issurim: ono’o and geneivas da’as. Ono’o is certainly of a Torah prohibition, and geneivas da’as is a Torah violation according to many. Furthermore, your sale may be invalidated on the grounds of mekach to’us.