QEII has a very distinct public image and so we tend to see familiar tiaras, necklaces etc time and time again. But we don't know what HM wears in private.
In The Queen's Diamonds there is a modern photo of Queen Mary's magnificent diamond sautoir which proves that the jewel is still intact. Perhaps HM wears it privately. It is versatile and, as was noted recently by 992234177, it would work well with HM's South African diamonds.
I strongly suspect that the very elaborate emerald stomacher created by Queen Mary still exists as QEII wears parts of it as separate brooches.
Hugh Roberts stated that the Teck crescents and roses tiara has been lent to the Duchess of Cornwall; yet, we have never seen photos of her wearing it.
As Queen Elizabeth the Queen Mother was not known to frequently dismantle and rework jewels, I suspect that her pearl and diamond sautoirs and her sapphire sautoir still exist and will re-emerge when fashions change.
(I suspect that the sapphire suite given to QEQM for her wedding by Queen Mary
was transformed into the elaborate sapphire sautoir worn by QEQM during the 1930s)
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