I'm not so sure that HM has designed the newer tiaras herself as its more likely to have been left up to Garrard to produce some marquettes. Maybe the results we see today were the best of the bunch. I guess it serves to highlight the supreme artistry and craftsmanship of say a Cartier or a Fabergé. They just don't make 'em like they used to!
The 1993 "Garrard the Crown Jewellers for 150 years" commemorative publication doesn't give much away with just this on the subject:
"When the Crown Jewellers provide new pieces for the Queen, current fashions inspire the design without making an aggressively contemporary statement which would quickly date. The Burmese ruby tiara ordered in 1973 is a perfect example. A conventional concept very boldly detailed..."
Geoffrey Munn merely states that Garrard "presented the Queen with the design".
In defence of the Brazilian Aquamarine Tiara, the main photograph that Sartorial Splendour chose to use wasn't exactly the most flattering they could find.
The tiara does have some fine and interesting detail...
For me, the aquamarine tiara shows definitively how incredibly difficult it is to incorporate big stones in a tiara. I think had Garrard not been 'constrained' in having to make such big aquamarines fit into it, I think it would've been more successful.
I personally find most tiaras that have to situate large single stones somewhat unsatisfactory. I'm so glad the Queen has never tried to fit Granny's Chips into a tiara. In theory, they would make an astonishing tiara (and I'd be fascinated to see what the top jewel houses would come up with if tasked with designing such a thing), but I just doubt it would work. On a smaller scale, the Dutch Stuart tiara has the same problem.
The brazilian aquamarine tiara is a favorite of the queen tiaras. Stones has an outstanding beauty, blue is deep. The position that uses the head and hair color can take your beauty. The necklace, earrings, bracelet, brooch, are magnificent art deco. It will be a favorite of Princess Consort Camilla.
It's the Delhi Durbar Tiara, currently on loan to the Duchess of Cornwall. The setting of the tiara allowed Cullinan III and IV to be mounted in it, as in your pictures. Cambridge emerald pendants could also be added on top. Only Queen Mary wore the Tiara with the Cullinans, and even she did it only a couple of times. She did add Cambridge Emeralds on top quite often though, especially when wearing it with the rest of her Delhi Durbar Parure.
Picture 1 - The Delhi Durbar Tiara in its current form (original)
Picture 2 - The Delhi Durbar Tiara with Cullinans III and IV (photoshop)
Picture 3 - The Delhi Durbar Tiara with Cambridge Emeralds and a Cullinan (photoshop)
Picture 4 - Queen Mary wearing the Delhi Durbar Tiara with Emerald pendants on top (original)
Picture 5 - Queen Mary wearing the Delhi Durbar Tiara mounted with Cullinans III and IV (original)
Picture 6 - The Duchess of Cornwall wearing the Delhi Durbar Tiara in its current form (original picture)
This would be a great starter set for DofCa. Very nice jewels, but not the big chandelier pieces like the rubies and Saudi pieces. Great for HM, but a little much for Kate. HM may wear this to show some affection for them, then pass them on to DofCa. The emerald choker might be a bit much to start.