Camilla wears the Greville tiara so I don't think the Teck Crescent Tiara is still available to her.
Of course arguments can be made as to why some tiaras are more likely to be worn than others.
I agree that there are jewels in the vaults that can be used to create a tiara as was the case with Sophie's wedding tiara, although I am not overly impressed with the tiaras that have been created during Elizabeth II's reign.
It will be interesting to see if and how things change with future monarchs.
ETA:
I am not 100% sure but I don't think that Camilla has worn the Teck Circlet as a necklace and am pretty sure that she has not worn it as a tiara.
Camilla wore the Delhi Durbar Tiara once and now wears the Greville Tiara, so like with the Teck Crescent Tiara I don't think that the Delhi Durbar Tiara is still available to her.
Margaret wore the Poltimore, Persian Turquoise, Lotus, Sapphire Bandeau and Cartier Scroll tiaras. The Poltimore was her personal tiara, a gift from her mother IIRC, and it's a guess, but since Catherine has worn the Cartier Scroll and Lotus tiaras, I suspect that the Sapphire and Turquoise tiaras are back in the royal collection.
I will update my list in the previous post to add the Cartier Bracelet Bandeau Tiara.