Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: “That should be calculated by the experts. So it should be taken to a jeweller so that he can examine it and see whether the gold reaches the nisaab (threshold at which zakaah becomes due or not).
If it does not reach the nisaab, then no zakaah is due, unless he has other gold which would bring the amount to the nisaab, then he should calculate the amount of gold that is mixed with diamonds, and pay the zakaah which is one quarter of one-tenth (2.5%).