I agree with Nellie, an emerald brooch is not the best match with a turquoise blue coat...
Anyway, it is definitely the Cambridge emerald brooch, worn without its usual emerald drop;
The Queen mostly wears this brooch with its pendant, but she occasionally wears it without the pendant:
12th January, 2014: divine service at Sandringham:
20th May, 2011: State Visit to the Republic of Ireland
2002: Visit to Jamaica:
And Princess Mary-Adelaide of Cambridge, Duchess of Teck (Queen Mary's mother) had worn the emerald and diamond cluster and the emerald pendant mounted on a stomacher, but separated from each other: