Interestingly, a corsage brooch in starburst design, looking quite similar to the Warwick sun brooch, but with a sapphire centre (and a few more diamonds on the knife-edge lines) appeared at auction a few years ago. I do not suggest it is Queen Mary's Warwick brooch (not even a modified version of it), but it probably gives us a good idea of the brooch's aspect. Definitely worth of a Queen!
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It's not possible because:
Queen Mary reportedly had the "Warwick" diamond sun broken up in 1911 and the stones could have been incorporated into new pieces, such as one her large stomachers
Queen Mary's Warwick Sun Brooch Wedding Gift