I donīt know if this might be the occasion, but the State opening of Parliament in 1909 took place on 16 Feb 1909.
From the Court Circular: His Majesty the King wore the Uniform of a Field Marshal. Her Majesty the Queen wore a black gown embroidered with gold and silver; a robe of ruby velvet embroidered with gold and lined with maneuver, and a Crown of Diamonds with Honiton lace veil. Her Majesty also wore the Cullinan Diamond, with Diamond and Pearl Ornaments. Orders: the Garter and the Kingīs Coronation Order.
The Court does not appears to have been in mourning, as the Princess of Wales wore a pale green dress.
--Previous Message-- : : The year would seem to be 1909 or early : 1910. : : Having looked at The Queen's Diamonds - : : The uncut Cullinan was sent for cutting in : January 1908 and the first nine were : produced by 13 October. : : I and II were reserved for the KIng. : : "...in 1909 they were temporarily : mounted as a pendant brooch for Queen : Alexandra's use." : : Alexandra does appear to be wearing : mourning. [Edward died 6 May 1910.] : : It looks like I was right about I suspended : from II. See pic below for size : comparisons. : : : : :