You can deduct the monthly payments (for the year in review) before calculating the amount of Zakah. However, you are not allowed to deduct the entire mortgage. If we do so, then no one would be paying zakah as most people have mortgages for homes or businesses. So, you are allowed only If the jewelry is for personal use you don't need to pay Zakah on it unless it is considered as excessive or extravagant. For details, please refer to the following answer.