At first thought, I would concur that the briolette drop is a pink topaz, since the central stone of Queen Mary's brooch (now integrated into the Duchess of Gloucester's necklace) is a topaz. It would be coherent to conclude that both jewels are made of the same stone, since they are worn together.

I still have a doubt about the briolette drop, though. Reason for that is that, whereas the cushion-shaped topaz of the necklace (and the other topaz jewels belonging to the Duchess of Gloucester) have a beautiful, translucid brightness, the briolette pendant looks a bit dull, opaque, milky... Therefore, I wonder if the pendant could be made of another less noble stone rather than genuine topaz (maybe smoky quartz?). The colour of the two stones would be close enough to wear them together without making a visual clash, but they would be different stones anyway.
(I insist it is just speculation from me, as I do not have any specific information.)

Here are two pictures of the Duchess of Gloucester on 20th June, 2018, at Ascot racecourses, on which the "dull", smoky aspect of the pendant is clearly visible:


(source: Mark Stewart's X account - if the picture does not appear, here is the link: https://pbs.twimg.com/media/DgJAKs3WkAUWjw0.jpg:large)


(source: Max Mumby/Getty Images)