Bryce, it is the Badge of a Knight Grand Commander of The Most Exalted Order of the Star of India (G.C.S.I.), which Lord Mountbatten received as Viceroy of India.

He usually wore it at the neck, since he wore the Garter riband most often.

The Gold star is visible in two of the portraits (second clockwise from top).

This was a lavish piece of insignia: the badge was formed of a cameo of a youthful Queen Vitoria, circled with the Order's motto, 'Heaven's Light our Guide'. It was always diamond-studded.

These insignia had been designed by Prince Albert.



Queen Mary's Sash Badge, with Queen Victoria's Star (all diamonds) and George V's Star of the Order of the Star of India .




Queen Victoria's Collar of the Order (RCIN 441295)




Badge and Star of a Knight Grand Commander of the Order of the Star of India (sold at Spink London)