When I compare again the picture of QEQM's necklace clasp and of QE2's topaz brooch, I agree that the sizes are slightly different. And the brooch has ten large diamonds, whereas the clasp seems to have only eight.
But the shape of both jewels remains strikingly similar: a central oval stone bordered by an immediate diamond-studded surround, then a larger surround of larger round diamonds with small interspersed brilliants. Could they be two jewels from a same set?
Regarding the possibility that the topaz brooch could be an inheritance from the Queen Mother: in 2011, GillW had posted on this site a recap of the occasions on which QE2 wore this brooch - and two of these occasions were before QEQM's death in 2002, the first one being in 1994:
"26-Nov-10 State Visit to Oman - Muscat, Al-Alam Palace 05-Sep-10 Balmoral - Canada's governor general 20-May-08 BP - Rec for voluntary services 21-Mar-07 BP - Dignitaries 11-Oct-06 BP - ambassador for Iran 07-Dec-04 BP - Party for "high flyers" 23-Mar-00 Aus - Immigration Museum In Melbourne 12-Apr-94 "Glebelands", Wokingham, Berkshire"
Unfortunately, I do not have any pictures from these two events in 1994 and 2000.