|The Perth Bridge Opening Brooch|
Amethyst Bouquet Brooch, which includes seven amethysts but does not otherwise match the description. The actual Perth Bridge Opening Brooch is instead the one pictured here.
At the Windsor Horse Show, 1995
The Perth Bridge Opening Brooch has only rarely been worn in public by The Queen. This is an intricate design, and the delicate sprays almost disappear, so it's not an easy identification. I suspect we haven't really seen it worn to its greatest effect.
Thanks to Franck for sending in information about this brooch!
Photos: National Library of Scotland screencaps, via Getty Images as indicated