Waheeda Abdullah, formerly known as Fatimah Lim, denies taking one blue diamond and one yellow diamond from Mariam Aziz in 2009.
The 35-year-old Singaporean national allegedly stole the jewels - both gifts from the sultan - from Ms Aziz's home in Kensington and Chelsea between July 1 and December 31, 2009.
The pear-shaped blue diamond is estimated to have a minimum value of $12.7 million (£8.1m) and the yellow of about $1m (£640,000). District judge Peter Clark remanded Abdullah in custody at Hammersmith Magistrates' Court in west London yesterday afternoon.
Abdullah, who is charged with two counts of theft, was ordered to appear at Isleworth Crown Court, also in west London, on August 27.
Ms Aziz had four children by the sultan before their 1981 marriage ended in divorce in February 2003.