Selling gold that contains zircon stones for the price of pure gold

Selling gold that contains zircon stones for the price of
gold is something that is subject to further discussion. 

If it is being sold for silver or paper money, there is
nothing wrong with that, so long as it is visible to the eye and the
purchaser knows about that, as you have mentioned. If it is being sold for
gold, then the stones must be separated from the gold so that the amount of
gold may be known, and it may be ensured that equivalent amounts of gold are
exchanged. This is indicated by the hadeeth of Faddaalah ibn ‘Ubayd about
the sale of a necklace, which was narrated by Muslim (1591) who said: When
the Messenger of Allaah (peace and blessings of Allaah be upon him) was
in Khaybar, a necklace containing pearls and gold, which was part of the
booty, was brought to him to be sold. The Messenger of Allaah (peace
and blessings of Allaah be upon him) ordered that the gold in the necklace
be separated from it, then the Messenger of Allaah (peace and blessings
of Allaah be upon him) said to them: “Gold for gold, weight for weight.”
According to another report: “It should not be sold until it has been
separated.” 

Al-Nawawi said in Sharh Muslim: 

This hadeeth shows that
it is not permissible to sell gold with something else for gold until it has
been separated, and gold should be sold for gold by weight, but it may be
sold for anything else however one wishes. Similarly silver combined with
anything else cannot be sold for silver… whether the amount is a little or a
lot. This was narrated from ‘Umar ibn al-Khattaab (may Allaah be pleased
with him) and his son, and a number of the salaf, and it is the view of
al-Shaafa’i and Ahmad. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was
asked about selling gold which is combined with zircon stones for gold of
the same weight. He replied: 

This transaction is haraam, because it involves riba, as it
is adding something to the gold by giving gold in return for the gems etc.
This is like the necklace which was mentioned in the hadeeth of Faddaalah
ibn ‘Ubayd, who bought a necklace that contained gold and pearls for twelve
dinars, and he separated it and found that it contained more, and the
Prophet (peace and blessings of Allaah be upon him) said: “It should
not be sold until it has been separated.” End quote. 

Shaykh Muhammad ibn ‘Uthaymeen.  

Majmoo’at As’ilah fi Bay’ wa Shira’ al-Dhahab.