A truly impressive tiara, thanks for the screen capture, LittleMo! And I actually didn't notice this stomacher. It looks like it is the Imperial Sapphire necklace, worn as a stomacher on the bodice.
According to Ursula's informative site, the tiara was sold at the 1970's auction of the late Princess Royal's estate. Read more here. And yes, it seems they were sapphires in the tiara/necklace .