For gold, zakat is due on the market value of the amount of gold one owns at time of the zakat due date.
Regarding your specific question, simply estimate the weight of actual gold for your jewellery (you can be “safe” in your estimate by assuming slightly more than your estimate, but not by too much lest it be excessive).
Don’t use “an average of 22 carat” but rather do each carat amount independently (21 carat, 22 carat, 24 carat, etc…).
And Allah knows best.
Answered By: Shaykh Faraz Rabbani