HRM is wearing the Imperial State Crown of Great Britain, which incorporates the "Black Prince's Ruby", a 140ct. Badakhshan spinel. The crown was modified for Queen Elizabeth II prior to the 1953 coronation.
In 2002 when Queen Elizabeth, the Queen Mother died at the age of 101 the crown was set on her coffin during her state funeral. Description from theenchantedmanor.com. I searched for this on bing.com/images