Fatimah Lim, 35, who accompanied Mariam Aziz as her personal assistant, allegedly had replicas costing £300 made of two large diamonds, and placed them in the safe of her employer's Kensington home before selling the real stones.
A jury at Isleworth Crown Court heard that Fatimah – who won a silver medal in Badminton at the 2002 Commonwealth Games – had run up debts while gambling with Mrs Aziz in London.
CLICK ON PHOTO TO ENLARGE
Between May 2008 and December 2009, she is accused of stealing three items of jewellery – a bracelet containing eight diamonds worth £3.35 million, a yellow diamond worth £600,000 and a flawless 12-carat blue diamond ring worth £7.7 million. She denied all three charges.
The Sultan of Brunei is one of the world's richest men. He was married to Mariam for 22 years before their 2003 divorce, and the couple have four children together. The former air stewardess, who remains on good terms with the Sultan, worth an estimated £12 billion, was described by prosecutors as "extremely wealthy" in her own right.
Jurors heard that Fatimah started working for Miss Aziz in 2003, firstly as her badminton coach and then extending her role to that of personal assistant and bodyguard.
The theft was only discovered in December 2009, when Miss Aziz asked her adopted daughter to take the diamonds, which had been set in rings, for resizing.
The trial, which is expected to last four weeks, continues.