Thank you for the delightful photos of the Duchess of Cornwall's choker, Arthur.
According to Roberts page 164 footnote 2 the large pink stone in the Gloucester tiara is a pink topaz.
I suspect that all the pink gemstones in the parure worn by the Duchess of Gloucester are pink topazes. Besides the detachable stone in the tiara she has 2 brooches, a bracelet and a necklace.
The second more elaborate brooch
Queen Mary appears to have had a collection of pink topaz jewels, one of which she gave to Princess Margaret.